Dodgeroll/Hit Resets
This is something you'll learn yourself if you start hardmode, but I never learned it myself in normal gameplay so I figured I would throw it in just in case. The idea is that when you hit a bullet or enemy with your katana, immediately after the the hit is registered, you can start a dodgeroll, and after you start dodgerolling, you can almost immediately hit something with your katana again. By doing this you can deflect a lot of enemies attacks and be unkillable for most of the time in combat due to the roll. Make sure you're not stabbing upwards with the attack, otherwise you'll fly up. and instead of dodgerolling you'll just enter a fastfall instead and will be killed.

Wallclimbing
This is only really useful during bunker part 2 to dodge a camera, if youfall off something during a speedrun and need to get up quickly, or (secret achievement spoilers) to grab the prism key on level 1. To do this press yourself all the way up to a wall and while holding whatever key is moving you towards the wall, press W, this will quickly push you far up a wall. You can't do this in midair, it is required to start off on the groun

Settings I use

• In the settings set your text speed to 2x, there's a lot of dialogue, even in speedrun action only mode, this will save you a lot of seconds.
  
• hitpause, 0% This makes it so there isn't a slowmotion effect when you hit something, this will save you a lot of time, it will take some getting used to though.
  
• Make sure you have skip rewind on, that way you aren't mashing your mouse after every level, you will still get rewinds during some level related scenes though, sometimes they can glitch out and you can still skip them though.

Another useful guide
This is for speedrunning, I could just copy the text from there and put it here, but that's plagiarism
https://www.speedrun.com/katana_zero/guide/r8h7k