Slow sensitivity. I play @ 1.
The aim, while camping should be about 1 meter away from the ambush point, so that you can get a kill without missing due to slow reflexes.
Always change position after killing, dont just stand there.
Learn how to shoot flickrs *scope open only for a millisecond then shoot* this will help you kill nearby enemies quickly.
Always buy a HE, so that when you shoot an enemy and it hits him on the legs, he does not die but HP is reduced to 16, quickly throw the HE to bang him off.
Buy a flash and throw it behind you, then ambush when you are sure that the enemy is gonna rush, this way, you will not get flashed but the enemies will be blind for a while!
Hope these are helpful