In the multiplayer games (not cooperation) the rocket launcher is probably the best weapon to use. It will only take one hit to kill, it can lock on if they are in a vehicle or using a turret by holding the right trigger and then releasing it, and it gives no time for retaliation. Of course, there is still a catch. If your opponent is far away and not in a vehicle, it is just about useless unless you can calculate how far they will go and how fast the rockets travel. Also, you only get two rounds per clip, so if you miss twice then you are wide open for a good five seconds or so which is just enough time for someone to kill you. However, there is an additional good side to the rocket launcher, you don't have to hit your opponent directly. As long as you hit the ground or something right, emphises on right, next to them, they will still die. Also, if you hit a little bit away from them, depending on how far away from them, it will still take out little to all of their shields, in which case you can pull out just about any other gun and kill them in few to one hit(s).