1.  If you are being chased by a greater enemy with more health and a better gun, you have to get out of there!  Try running away through a corner, then when your friend turns the corner after you, go back the way you came to confuse him!
2.  Get the body armor!  Each level has its own spot where the body armor can be found.  It's like having and extra life.  The great thing about body armor is that you can pick it up after you've been shot unlimited times!
3.  Get the good gun!  Same as the body armor, the best gun or weapon is always found in the same spot.
4.  If you keep getting found and chased, and number 1 doesn't help you, turn off the radar.  It is one of the time cheats and isn't that difficult to get.
5.  Try shooting your enemies in the head!  It does tons more damage than shooting him anywhere else.  If you shoot him twice in the head, you might be able to kill him already.
6.  In most areas, there are places where you can snipe your enemies from above, especially in the Library/ Stacks.  This makes it hard for your enemy to see you since you're above his line of vision.
7.  If you know that your enemy is going for the body armor, get there first and take it, hide near it, and then when your enemy comes to get it, you can ambush him and have full armor!

8.  Close doors on you enemies!  If your enemy has to open a door to get to you, wait for him to open the door, then close it on him!  This is very effective and I use it all the time against my brother.  (it really makes people mad)

9:  Destroy the body armor:  If you already have the body armor and you want to ensure that your enemy won't get it, shoot the body armor with your gun until it looks something like roadkill, but not until it falls apart.  Once it's destroyed, no one can pick it up, and it seems to take forever to regenerate!