Many people try to use a combination of the Plasma Pistol(PP) and Battle Rifle(BR) to down your shields quickly and then a quick headshot.
It's a powerful combo, but not without it's weaknesses. If you can avoid the first PP shot, then your enemys advantage thins. While they charge their plasma pistol, they glow a cool green, making for easy detection. Simply keep alert, checking all the major locations from which you are often killed. Simply sidestep the blast, or hide behind an object.
After you waste the opponents PP, he still has his BR to fight you with.
So, you now have to counter his BR. If you have a BR, you can still get the first shot if he's still swithing guns. A grenade is also effective, if you can aim it well. If you are too far and have no way to stop the BR, run like a wussy, and try and find better weapons. In a team game, use your teammates to help, but in a FFA game, simply look for easier prey as you search for a better weapon