This guide may contains spoiler. I don't really know if it actually one, so proceed with own risk.
This guide also my very first guide in steam, because I usually the one that seek guides instead. Additionally, I'm still not getting used with steam formatting system, so I might get the format wrong.

Finally, as I write this, I've just finished fighting her so I don't know the rest of the boss to compare with.

So uhh.... Let's get straight into the point.

Remilia is, so far by my progress, one of the easiest boss you'll ever fight (aside from Cirno). Compared to Kokoro, she's not really that hard (and no long animation before death like Kokoro as well, that one took twenty years to finish). She heavily relied on patterns and doing it repeatedly. What makes her one of the hardest is, in fact, her damage output is so high that it could straight kill you in one or two hit (and other factors that might affect how hard she is, will be shown later).

This made me dead inside tho.

The reason why there's a trick to quickly win by using Yuyuko (DLC) is because this consistent pattern can't be miss.

Remilia ONLY has these pattern, and will have nothing else:

• Circle
  
  
• Arrows (Can reflect on walls)
  
  
• Leap with base 3 leaps and will increase by one for each phase and always zigzag
  
• Gungnir with base 1 gungnir and will increase by one for each phase, picture is the cone sight along with the circle
  
  
• Teleport (Always to the middle of the stage)
  
• Large AoE (Phase 2)
  
• Leech Leap (Phase 3, heals her HP)
  
• Return (Repeat the pattern of the phase from beginning)

These are the pattern for each phase:

Phase 1 (HP 100 - 75%):
Circle
Gungnir
Leap
Arrow
Circle
Leap
Gungnir
Circle
Gungnir
Leap
Arrow
Circle
Gungnir
Teleport
Return

Phase 2 (HP 75 - 50%):
Large AoE
Gungnir
Leap
Circle
Arrow
Leap
Gungnir
Circle
Teleport
Circle
Return

Phase 3 (HP 50 - 0%):
Leech Leap
(Arrow twice (first throw is thrice)
Leap) x3
Teleport
Circle
Gungnir
Return

Play with Marisa with full upgrade of her and try to use both attack at once (makes her spark larger but also reduces stamina).

Personal Tips:
If you ever played Dark Souls, play like it. You know how because it's basically the same (dodging patterns).
Otherwise, here are the tips:

• Her gungnir is avoidable, but better move normally (don't dash) because it might becomes homing. She rotates according to your position, but Marisa is fast enough to make her can't catch up and miss throw.
  
  
• Gungnir has a cone sight, but the more further you are, the more likely it'll become homing. So stay closer when she's going to homing.
  
  
• Try not to get damage when she's doing gungnir. Because you'll be staggered for around half duration of her gungnir and sometimes will homing even if you already away from the cone sight (It's One Hit KO with gungnir if you get damage before as well).
  
  
• Every phase ALWAYS make her teleport into the middle first, even she's already teleport in the phase (she'll do it first before breaking to the next phase).
  
  
• On second phase, you can carefully avoid the Large AoE and deal normally or throw Marisa's bombs (as much as possible) to the middle. This is risky because you'll get damage nearby her and her Large AoE skill, quickly dash after putting the bombs. It'll quickly deplete her HP to 50% or lower.
  
  
• If you're good at looking at reflection or something, arrow pattern should be easy enough. But beware of phase 3 arrow pattern, as it spams a lot than usual. Tips is that she'll throw averagely around your character, so you can dash when it's nearby. Always careful to see her leap.
  
  
• When she LEECH LEAP, take the corner of the room, her leap should always end up to the left (from back perspective) and never reach you. She'll have a small idle time before doing arrow afterwards. My usual spot is the bottom right corner as it's easier to see her come, sometimes bottom left, but never upper.
  
  
• Unless you know where to go, NEVER attack Remilia on phase 3 UNTIL she does gungnir. This is the only best time for you to hit her because less bullets on the screen.
  
  
• Watch your stamina and git gud.

These are several factors that I thought why she becomes one of the hardest boss in the game:

• Her damage output is mostly big.
  
  
• Her stagger is, personally, quite long. Long enough that you can compare her to Dark Souls meta, Giant Dad (aka, she could keep staggering you until you died).
  
  
• Her dress color is red, matching with the mansion color and could make a camouflage.
  
  
• Marisa dress color is black (if you use her), also quite matching with the mansion secondary color and could confuse your position if you focused on looking the bullets.
  
  
• This door colision.
  Screw this door.
  
  
• Patience (otherwise, you'll be greedy and died like a loser like I did for two and a half hour straight)

That's all, thank you for reading this guide. m(_ _)m
Yay!