The Impulse Rifle is a one-shot kill anywhere on the body. To get it, go to the top floor using the center lift and jump off the end nearest the Impulse Rifle. You should just make it. Immediately jump off there because your enemies can easily see you.
Get an easy win by gaining the Rocket Launcher from the first floor. Then you can just stand on the top floor and keep shooting at your opponents from the side nearest the lift (not the middle one). Enemies spend most of their time there.
There is Ballistic Armor available when you go to the top floor and jump off the edge nearest the doorway. Go through there and the armor will be on the right-hand side. There are extra rockets to the left.