When playing on Xbox Live, have a boomshot and a sniper in hand. When your enemy is taking cover, fire two boomshot bullets as fast as you can. They will think you are out of ammunition, so take out the sniper and aim at their position. As soon as they come out, get a head shot.
When a player is teasing you by ducking up and down behind cover, time his ups and downs. Just before he ducks, shoot but purposely miss. He will then come up again. When he does, quickly blast him.
Put away all your guns and say that you have nothing remaining--but have a sniper or other good weapon ready. Most of the time they will come for you. Try to spot him and shoot him in the head.
"Roadie Run" directly at your opponent with your shotgun, and when you get close (a few feet away), release A and immediately shoot. You can get some easy kills by doing this. It works best if you sneak up on your enemy from behind.
On any small multiplayer map (like Mansion), to know where the enemies are located, go inside the door and throw a smoke. Other people may say something like "Incoming smoke!", thus letting you know that the enemies are near.