Hopefully, you've raised your levels a bit since the first ten floors, because the next ten floors and every one after that shall become increasingly difficult. To start off, let's introduce you to monsters that hide in the storageroom treasureboxes. They get first attack and are somewhat fast. Thier first attack can do up to 400 damage, so you should heal immediately if possible. Also, the monster is weak against tranquilizer rounds, so if you have them, take advantage of them.

On the 20th floor, you will meet a pair of aligators. They're similar to the one at the beginning of the game, but with only one target each. These shouldn't be too much of a problem so long as you take one at a time(or if you have burst effect, you can try to hit them both at once). Win the battle, and you get another key.
  

Items to be found

Super Tool Club 4 Defense+?	Tool Monster
PE+? Sp Suit 1	Bullet Cap+? Tool M96
Tool Kv Suit 1	Rocket Offense+? Kasul T-card
Range+? Full UZ B Hawk T-card	(2)Tool M203-5
P229 AM44	Defense+? Cr Jacket