There is no other game like Team SWAT in Halo 2 that evens the playing field more. Learn a few basic techniques and skills and then compete with the best.
Anticipate Number 1: Set your look sensitivity (the controller option that adjusts how fast you turn) to level 3. You may even want to start off with level 2. This will allow greater control of your weapons, so that you can hit someone in the head on the first shot. Chances are you won't have time to get many shots off.