Getting 5 skulls in the first-half of this section is pretty easy. Just keep an eye on enemies especially Kobold Runners and clear every wave fast to meet every completion time in this section.

This is a free 5 skulls stage. Just put any traps that you like. The only tough enemy in this stage is the Armored Ogre. If you want a challenge you could try my strategy here by only using crossbow + skull Anatomy upgrade.
Youtube Link: Orcs Must Die(Only crossbow + Skull Anatomy Upgrade) The Hallway(Nightmare) 5 skulls walkthrough

Another easy 5 skulls stage. Just put any traps that you like properly. If you want a challenge you could try my strategy here by only using crossbow and spike trap. try it, It's pretty fun :D
Youtube Link: Orcs Must Die(only crossbow and spike trap) The Fork(Nightmare) 5 skulls walkthrough

Another free 5 skulls stage. Just put any traps that you like properly and 5 skulls are already yours. If you want a challenge you could try my strategy by only using crossbow and spike trap.
Youtube Link: Orcs Must Die(only crossbow and spike trap) The Corner(Nightmare) 5 skulls walkthrough

My strategy here is simple. Use wind belt to push enemies off to the toxic water or we could call it the "bath" water. If you don't like my strategy, I recommend slow traps like Tar Trap or the Ice Amulet to slow down Gnoll Hunters. In my opinion, every trap works here. Just put it properly.
Youtube Link: Orcs Must Die The Baths(Nightmare) 5 skulls walkthrough Let orcs have bath in the Baths!

There are two passages in this stage, which leads enemies to the rift and we need to block one specific passage which is very narrow and hard to put traps on. My strategy here requires us to have a barricade trap. Kobold Runners are the main problem here. Be sure to have an instant-hit trap in your arsenal like brimstone and zapper. Tar trap and Ice Amulet can help slow down runners, Ring Of Lightning is also helpful here, it can instant-hit kobolds and kill them.
Youtube Link: Orcs Must Die Runner's Alley(Nightmare) 5 skulls walkthrough

Just like in the previous stage, this stage has two paths which lead enemies to the rift. Just barricade the narrow passage on the side and setup a chokepoint in another passage. This stage is kinda easy, So, any traps would work, I think. Since there are kobolds in this stage, I recommend an instant-hit trap/weapon like Brimstone, Zapper and Ring Of Lightning.
Youtube Link: Orcs Must Die Overpass(Nightmare) 5 skulls walkthrough

One of my favorite stage in the game. There are two passages in this stage and in my strategy we need to defend both sides. Guardians are really helpful here especially the Paladin Guardian. I find barricades not effective here because of the kobold sappers or I would like to call it "Kobold Bombers" or "Kobold Terrorists". These kobolds are very effective against barricades and sometimes, they prioritize the barricades over you. Also, the passages are not connected in the first-half of the map. So, barricading in the first-half of the map is not a good idea because when there are multiple paths and they're not connected to one another, Enemies have no way to go, therefore, Their only choice is to destroy the barricade in front of them. For traps recommendation, I recommend traps/weapons which have fire attribute like Flame Bracers or Brimstone to counter those pesky Frost Ogres.
Youtube Link: Orcs Must Die Twin Halls(Nightmare) 5 skulls walkthrough

Pretty tough stage. Ceiling and wall traps are not effective as they're used to be in this stage, floor traps, on the other hand, are very effective. In my opinion, Floor Burner + Tar Trap is a good combo here. The passages in this stage are narrow, so, A trap that can attack enemies in a straight line or in an arc is really good and Floor Burner does that kind of job. The problem with Floor Burner is that it has a little bit of delay before firing.Sometimes, Kobolds take advantage of that delay and outrun the Floor Burner before it fires. That's where the Tar Trap comes into play, Tar Traps slow enemies, It gives the Floor Burner a little bit of time to setup before enemies outrun the Floor Burner.
Youtube Link: Orcs Must Die Lunchbreak(Nightmare) 5 skulls walkthrough

This stage has similarities with "Lunchbreak" stage, Ceiling and wall traps are not effective here. Floor traps, on the other hand, are very effective. Unlike in the "Lunchbreak" stage, this stage introduces us to flyers: one of the most irritating enemies in the game. Archers are really good in this stage and also the Ice Amulet since the types of flyers here are not immune to ice, Ice Amulet can freeze flyers which are not immune to ice and smash them onto the ground or drown them in a pool of acid or lava. Killing them instantly.
Youtube Link: Orcs Must Die The Stream(Nightmare) 5 skulls walkthrough

Another tough stage and also the last stage in act #1. In this stage, we need to defend two rifts which are facing each other and we need to defend two enemy entrances which are located on opposite side. One is located at north and one is located at south. In my strategy, we need to setup barricades first on the right side hall that faces the north enemy entrance. Then, setup traps in the vicinity of the north entrance during wave #1 up to wave #3. When wave #4 comes, go to the south entrance and setup traps there. I like to put Paladin Guardian in each entrance. Guardians get enemies attention. They can stall enemies for a period of time which is enough to defend two opposite entrances of this stage. In wave #5, there are more enemies in the south than north, so, better put some traps in the south in this period. Be careful with sappers in the north though. When wave #6 comes, both entrances should be decently fortified. From this point, continue to fortify the north more and fortify the south less because most powerful enemies will come from north.
Youtube Link: Orcs Must Die Sludge Hole(Nightmare) 5 skulls walkthrough

In this Act, Expect more waves in each stage, myriad of enemies and tricky but fun stages.

Pretty tricky stage. Kobolds and Gnoll Hunters are the main problem in this stage. Using guardians here can be tough because this stage has a lot of Gnoll Hunters. If possible, kill those Gnolls fast before they kill your guardians. My run here was kinda messy and I almost lost a rift point. Brimstone, Floor Burner,Tar Trap and flame bracers are good here. Those traps/weapons are good with kobolds and counter frost ogres which are present in this stage. Also, bring a hard hitting ceiling trap like Swinging Mace and Pounder.
Youtube Link: Orcs Must Die Chaos Chamber(Nightmare) 5 skulls walkthrough

Pretty easy stage. There's one very good "chokepoint" in this stage, Just properly put any floor traps that you like in that "chokepoint" and 5 skulls are yours. I recommend Floor Burner, Tar Trap, Ice Amulet, Ring Of Lightning for primary traps/weapons. Add Archers and/or Ballistas for supporting fire.
Youtube Link: Orcs Must Die Chokepoint(Nightmare) 5 skulls walkthrough

This is a tough stage. There are two rifts and two entrances to defend. In the first and second wave, fortify your defense in the north entrance. In wave #3, Fortify your defense in the west entrance. In wave #4, put some(at least 2) archers near the rifts. These archers could kill stray kobolds that are getting closer to the rifts. If you still have money, put defenses to the west entrance. In wave #5-#10, Gradually fortify your defense on both sides. Note: Don't rely on archers to kill kobolds, you should pay attention to those kobolds and kill it yourself if possible. Also, Protect your guardians from gnolls.
My strategy here requires Guardians, Ice Amulet and Ring Of Lightning. For traps, I recommend Tar Trap, Floor Burner and Brimstone. Also, add any wall traps that you like and put them near the entrances.
Youtube Link: Orcs Must Die The Arena(Nightmare) 5 skulls walkthrough

There are three paths here, two on top and one at the center. In wave #1-#2, put defenses in the center path. This path is going to be our chokepoint. In wave #3, block the two entrances on top by using barricades. Flyers and kobolds are the main problem in this stage. I recommend Grinder, Tar Trap and Ice Amulet in this stage. Grinder is good for low hp enemies like kobolds and little flyers or I would like to call it "flylings". There are lots of kobolds and flylings in this stage that's why Grinder is a good trap here. Also, flyers in this stage tend to move to lower ground near walls, which gives Grinder a chance to grind those flyers. Ice Amulet is also very good here, There are no flyers that are immune to freeze in this stage, so, you can instantly kill all flyers here with your Ice Amulet. Also, when flyers come to lower ground you can use the right click ability of Ice Amulet and obliterate a bunch of flyers.
Youtube Link: Orcs Must Die The Balcony(Nightmare) 5 skulls walkthrough

This stage is fairly easy. There are tons of Frost Ogres in this stage. I recommend traps/weapons with fire attribute like Brimstone, Floor Burner and Flame Bracers. Mushrooms are also good in this stage.
Youtube Link: Orcs Must Die The Tower(Nightmare) 5 skulls walkthrough

This stage is tough. There are two rifts to defend on west and east and there are two entrances in north and south. There two ways that I know to complete this stage. Either you focus your defenses in one of the rifts and put less defense on another or distribute your defenses on both sides. In the first strategy, you need to deal with the enemies that is going to the rift with weak defense and hope that your defenses in another rift are good enough to handle enemies without much of your help. In my run, I've used the second strategy. To do this strategy, we need Guardians to stall enemies and gradually build up your defenses on both sides. Also, try to shoot those metal pots above when enemies are close to those lava puddles that you see on the ground. My run here was messy and I forgot to utilize those metal pots. Though, I still managed to get 5 skulls in this stage. I recommend brimstone and tar trap for kobolds and Ice Amulet for Fire Ogres. Also, I recommend Ring of Lightning which is good for minion orcs and kobolds.
Youtube Link: Orcs Must Die The Library(Nightmare) 5 skulls walkthrough

This stage is kinda tough. Kobolds Sappers and Flyers are the main problem here. There are two paths here that lead enemies to the rift. Choose which path you want to defend and barricade another path. If your barricade is destroyed, replace it immediately. I recommend brimstone, archers and storm zapper in this stage. Since the paths in this stage are narrow, Floor Burner is good here. It can fry a bunch of orcs in a single fire. Unfortunately, there are fire ogres in this stage. Those ogres are immune to fire and traps with fire attribute are not very effective to them. To counter those ogres just bring Ice Amulet.
Youtube Link: Orcs Must Die The Bridge(Nightmare) 5 skulls walkthrough

Note: I made a mistake in my video. Don't put barricades in the first hall either on left or right side. Enemies will destroy those barricades instead of moving to a different direction.
This stage is kinda hard in the first-half but once we have a solid defense, this stage is gonna be easy. This stage has three paths the left,center and right paths. In my strategy, the right and center paths are blocked. In my strategy we need Barricade, RoL, Ice Amulet, Zappers and Brimstone.
In the first wave, put barricades in the second hall on the right. Also, put two barricades in the center hall near rifts. Enemies will come to the left, defend the left side using your weapons.
In second wave, put some defenses in the second hall on the left side. Enemies will come to the right this time, try to kill enemies near the center before they come to the left side.
In third wave, put two barricades again in the second hall on the right side, add some defenses and clear the wave.
In fourth wave, Put brimstone or zappers in the second hall on the right side and clear the wave. If one of your barricades is destroyed replace it immediately.
In wave #5-6, both sides should have a decent defense.
In wave #7, put traps in the first hall on the right side.
In wave #8-9, add more traps to both sides.
In wave #10 add traps in the first hall on the left side.
In wave #11-#12, add traps in the first hall on left and right side.
Note: To prevent gnolls from coming in different directions try to position yourself in the center-left that is near to the first hall on the left side for a period of time. This makes gnolls move to the left side instead of moving at different directions.
Youtube Link: Orcs Must Die Killing Fields(Nightmare) 5 skulls walkthrough

This Act consists of 6 stages. Some stages are hard some are just fairly easy to finish. This Act is also the last Act in the game.

This stage is pretty easy. we just need to follow a certain pattern. We need to defend three entrances here at north, west and east. I recommend Swinging Mace, Guardians, Ice Amulet, Tar and Floor Burner. For wall traps, any wall traps will work just fine here.
In first and second wave, put traps in the hall near the north entrance.
In wave #3, setup tar traps to west and east halls.
In wave #4, setup Paladins to west and east halls then get the Guardian's regen upgrade.
In wave #5-#6, add defenses to west and east.
In wave #7-#10 Fortify your defense in all directions
Youtube Link: Orcs Must Die Rebirth(Nightmare) 5 skulls walkthrough

This is a tough stage. There's one chokepoint at ground level but flyers don't go there they fly at top floor to reach the rift. So, we need to defend both ground and top level. I recommend Archer, Ballista or zapper, Flame Bracers and Ice Amulet. For other traps, you can bring your own trap combination or if you're lazy just copy my traps in the video. Ballista is really good here but it's expensive to setup. So, I go with zappers in my run.
In wave #1-#3, add defenses in the chokepoint on ground floor.
In wave #4-#7, add archers and zappers on top floor. For archer placement. It's best to place the archers near the rift. Placing them in the balcony above the ground chokepoint or placing them near the railings will cost their lives. Placing my archers in those places was a pretty bad mistake in my run.
In wave #8-#12, Fortify your defense in all directions.
Youtube Link: Orcs Must Die Gateway(Nightmare) 5 skulls walkthrough

Pretty hard stage. This stage has two rifts, two ground pathways and a balcony at top where flyers can enter. I recommend Archers, Ice Amulet, Ballistas, Tar and Floor Burner. First, block the east pathway and let the enemies go to the west pathway. There are flylings present in the first wave, so, pay attention to them. In this stage, there are guardians that are already stationed, which is good. I think the archers in the balcony on the ground level are badly positioned. So I let them perish. In this strategy, Trying to save those is a hassle.
In wave #2-#3, add archers or ballistas at the back of the hall near the balcony at top.
In wave #4, block the pathway in front of the balcony on ground level.
In wave #5-#13, Fortify your defense at top and on ground.
Note: Prioritize flyers if you have a chance. Also, pay attention to gnolls. They can kill your archers weakening your defense against flyers.
Youtube Link: Orcs Must Die Hard Climb(Nightmare) 5 skulls walkthrough

This stage is kinda hard. Just follow my guide here and we'll be fine. I recommend Guardians, Ice Amulet and Zappers. For other traps, You can make your own trap combination or just copy my traps in the video. This stage has three floors. The rift is on the 2nd floor and there are entrances on top and ground floor. Each entrance has a nearby chokepoint.
In first wave, enemies will come from top. Setup your defenses in the nearby chokepoint on top and get the Guardian's regen ability.
In second wave, enemies will come at the ground entrance. You may not have enough money to setup defenses on ground So, try to kill enemies using your weapons.
In third wave, prioritize kobolds on top and ground levels. Then, setup few defenses in the nearby chokepoint on ground level.
In fourth wave, setup defenses at the top chokepoint and also put zappers on a ceiling near the flyers entrance.
In fifth and sixth wave, setup defenses at the ground chokepoint. In this wave, both chokepoints should have a decent defense.
In wave #7-#12, Fortify your defense further.
Note: Use those hanging spiky rolls to your advantage.
Youtube Link: Orcs Must Die The Squeeze(Nightmare) 5 skulls walkthrough

Pretty fun stage. I recommend Ice Amulet, Flame Bracers, Floor Burner, Swinging Mace,Paladin Guardians and Arrow Walls. There are two rifts in west and in south, Two entrances in north and chokepoints near entrances.
In first wave, enemies will come on the left and right side in north, put traps in the chokepoint on the right and kill all kobolds. Then, move to the left side and kill all orcs using your weapons.
In second wave, put traps in the chokepoint on the left and kill all kobolds. Then, move to the right and see if there are orcs that pass through your traps there. Also, get steel weaver first upgrade.
In third wave, put a Paladin on the right and defend the left. Often check the right for stray orcs/kobolds. Also, build up your defenses on the left and get the Guardian regen upgrade.
In fourth wave, put a Paladin on the left and build up your defenses on both sides. There are gnolls here, defend in which side the gnolls go. If they go right, go to right, otherwise, go to left. In my case, they went right so I went to right.
In fifth wave, build up your defenses on the right and defend left.
In sixth wave, Defend where the gnolls go, either left or right. Gnolls like to prioritize the player, so, they will probably go where you are. You're in the left chokepoint, so, gnolls will probably go there. Also, build up your defenses on the left. After this wave, two chokepoints should have a decent defense.
In wave #7-#12, Fortify your defense on both sides.
Tip: Use those hanging spiky rolls to your advantage.
Tip: If you see a Paladin having a hard time with ogres or gnolls and you're on the other side, try to help them by using your crossbow's stun or Ice Amulet if an enemy is not a Frost Ogre.
Youtube Link: Orcs Must Die Stairs Of Doom(Nightmare) 5 skulls walkthrough

Note: I can't write an in-depth written guide in this stage because of the character limit per section. Better watch the video to understand my strategy here.
Oh boy... We're in the final stage. Finding an effective way to complete this stage without losing a single rift point is really hard. Regardless, There are multiple ways to finish this stage flawlessly. I recommend Ice Amulet, Flame Bracers, Swinging Mace, Tar, Guardians, Floor Burner and a dummy. You can use mushroom as an alternative to dummy but it's more expensive than dummy. For wall traps, any wall traps would work but I think wall arrow is better here because it has more range than other wall traps. In my strategy, we need to follow a certain pattern. First setup defenses at north, then at east, at south and at west. Also, don't forget to use those hanging spiky logs on top. We're in wave #5 after that, so, setup archers on top kill two ogres fast while paying attention to orc archers. In wave #6, go on top and kill kobolds and gnolls. In wave #7-#11, fortify your defense in all directions. In wave #12, put three dummies at north and at south; kill all ogres fast.
Youtube Link: Orcs Must Die Finale(Nightmare) 5 skulls walkthrough

If you want to fully upgrade all of your spellbook's traps/weapons/guardians then you must get 5 skulls in all stages in this section.

This is pretty easy. We need to defend at west and east. In the first wave, you'll be spawned in the west. Save your money and kill all orcs by using your weapons and those acid pots at top. In the second wave, enemies will come from east, put traps in the east hall near the rift and kill all enemies.
In third wave, put more defenses in east hall, kill kobolds that come from both sides, clear enemies in west and go back to east. In wave #4-#7, repeat the steps in third wave. After wave #7, east hall should have a solid defense. In wave #8-#9, start to build up your defenses in the west hall. In wave #10-#12, fortify your defense in both halls especially the west hall.
Youtube Link: Orcs Must Die Lost Adventures: Double Trouble(Nightmare) 5 skulls walkthrough

Remember "The Tower" stage in Act 2? This stage is the same as "The Tower" stage in Act 2 but it's reversed. In my opinion, this is much easier than the original tower stage. In this stage, we will encounter a new type of flyer: The Ice Flyer. This flyer is immune to ice, so, we can't instantly kill this with Ice Amulet. We will still use Ice Amulet in this stage. It may not be good with Ice Flyers but it's useful to other enemies like Fire Ogres and Gnolls which are present in this stage. In this stage, I recommend Ice Amulet, Fire Bracers, Archers; get the fire arrows upgrade and Ballistas. The strategy here is to put archers near the rift, get the fire arrows upgrade, setup a solid defense in the chokepoint near the first stairs and put Ballistas on the 1st floor ceiling.
Youtube Link: Orcs Must Die Lost Adventures: Reverse Tower(Nightmare) 5 skulls walkthrough

This stage is hard at first but once you build up defenses this is easy. So, this stage has two halls at opposite sides and one rift. The strategy here simple, Fortify the west or east hall and it should be decently fortified after wave #3. Next, fortify both sides especially the hall with weak defense. In wave #1-#3, pay attention to enemies especially kobolds and get the enemies attention if possible. This part is the hard part since you have less money in these waves. I recommend Archers, Ice Amulet, Ring Of Lightning, Brimstone and Tar.
Youtube Link: Orcs Must Die Lost Adventures: The Pit(Nightmare) 5 skulls walkthrough

This is a pretty tricky stage. This stage has three pathways. In my strategy, we block the right and middle pathways. Then, build up defenses on the left, Also put some brimstones near the entrances to preempt sappers from wreaking havoc. Sometimes, sappers may destroy your barricades. If this happens, replace it immediately.
Tip: In wave #1,#4,#8 and #12, try to stay near the upper stairs on the left. It may help divert the sappers attention to you instead to your barricades.
Youtube Link: Orcs Must Die Lost Adventures: Triple Down(Nightmare) 5 skulls walkthrough

In my opinion, this is the toughest stage and much tougher than Finale. There are tons of Frost Ogres and Ice Flyers in this stage, fire attribute traps/weapons like Floor Burner, Brimstone and Fire Bracers are really good here. In high level waves, Frost Ogres and Gnolls are prevalent. I recommend mushrooms to charm some of those instead of fighting them. Also, don't forget Barricades because this stage has multiple pathways that we need to block. Also, bring archers, some ceiling and wall traps that you like.
In first wave, block the 1st hall west entrance in the east near the enemy entrance and then block the 2nd hall east entrance in the west near the rift.
In second wave, collect money, clear enemies and get ready for wave #3.
In wave #3, block the 2nd hall west entrance in west, kill gnolls then go near the rift and kill all flyers. Use your weapons here efficiently. Also, don't let Ice Flyers freeze you. When you're near the rift, it helps regenerate your mana slowly and your health nicely. You can block the 1st hall west entrance in west instead of 2nd hall west entrance but enemies could overwhelm the left pathway to the point that they become unstoppable force. Blocking the 2nd hall west entrance in west eliminates that problem. If you really want to block the 1st hall west entrance in west, I think you may need to modify your trap set if you completely copy mine.
In wave #4, add defenses to the left, put archers and kill flyers while paying attention to enemies on land.
In wave #5, just defend the left pathway, there are no flyers here. Also, start adding defenses in the 1st hall in west.
In wave #6, Setup a mushroom at the right side near rift, Kill the the Frost Ogres at the start then go near rift and start killing flyers. Three gnolls will come at you at the same time and dealing with those are tough 'cause at this point, you're dealing with flyers, gnolls and orcs, so, wait near rift, charm one gnoll then kill the two. Pay attention to your surroundings for stray enemies coming from left pathway.
In wave #7, Add more defenses in the 1st hall in west, setup a mushroom near the 1st mushroom that we set up lately and clear the wave.
In wave #8, setup mushrooms below the stairs in 2nd hall in west. There are multiple Frost Ogres here that will come to the rift at the same time. Reduce their force before they come to 2nd hall in west.
In wave #9, setup a mushroom near the two mushrooms near the rift. At this point, you have three mushrooms near rift. Three gnolls will come at you, we don't have time to deal with those, so, wait near rift and charm them.They also help stall flyers and ogres for a period of time. Then, add more mushrooms in the 2nd hall entrance near the left side of rift and add one more mushroom near rift at north.
In wave #10, Add some defenses and mushrooms near the left side of the rift and add one more mushroom near rift at north
In wave #11, repeat the steps in wave #10 and reduce kobolds numbers at the start before they come to the 2nd hall in west.
In last wave, pairs of gnolls will come at you with brief intervals, you should have at least 5(I prefer 6) mushrooms near rift at north. Wait for gnolls to come and charm them all and kill any that you failed to charm. Don't forget to fortify your defense in the left pathway and pay attention to both north and left pathways.
Youtube Link: Orcs Must Die Lost Adventures: Great Gorge(Nightmare) 5 skulls walkthrough