Try the springfield, its the easiest.
Keep stopping power on for a usually constant 1 hit kill,
Either Betties if you like to stay back, bandolier if your good, bomb squad if you rush alot.
If you see an enemy, stop moving, aim at the enemy, shoot and move at the same time.
I like to shoot and move back.
Theres the one where you crouch and shoot at the same time, and double tap Triangle and shoot.
Timing is everything,
If your on PS3 add xxOutkast and we can practice in private matches.