While finishing the campaign multiple time in different difficulties i took some notes on boss fights. They're my least favorite part of the game, they're not reliable and I hate to have to go back 10 floors under only because of my poor upgrade choices. Thought i'd share what i gathered for anyone interested.

Disclaimer

• i ain't an expert at this game, feel free to correct me. (and sorry for the english)
  
• i don't recommend this guide to live a more challenging and full experience, it's mostly meant for full completionner and noobs. (like me)
  
• minor spoilers ahead

The following advices don't include the final boss (you can scroll down to the last segment) and all levels labelled "no upgrades needed", in the later you can focus on only level appropriate upgrades, it's a win-win!

• the basic uprades you need to take regardless of the boss are Boss Killer, Critical Hit, Slow Motion and Toxic Smell
  
• dodge any shiny stuff, never stop shooting and you'll get it
  
• if you see big red eyes, shoot them
  
• i personnally would'nt recommend the katanas but don't take my word for it, use the weapon you're most comfortable with
  
• the Antiacid and Lava Boots upgrades are overpowered in the concerned arena, i wouldn't recommend it if you wanna keep a bit of a challenge

• basic boss upgrades only

• basic boss upgrades
  
• acid arena

/!\ be cautious in melee

• no ugrades needed

• no upgrades needed

• basic boss upgrades only

• basic boss upgrades
  
• lava arena

• no upgrades needed

/!\ no upgrades avalaible, only weapons

• same advice as before, use the weapon you're the most comfortable with
  
• use the hiding spots, beware of rockets and keep shooting
  
• you can sometimes find a soda can next to the entrance
  
• a bit long but not too hard once you get the hang of it

Congrats, you can now continue following this guide for another difficulty or leave to the endless mode. Go touch some grass!!!