You know those pesky levels like
battle creak where you start out with that useless plasma pistol? Well it's not
useless for these things.
Easy Kills: Charge up a plasma shot and charge the other player. Once close
enough to have a guaranteed shot fire, then keep running toward your
opponent. Though your plasma pistol shouldn't be able to fire anymore, if you
whack the other player while his shields are still down, it kills him.
Tossing grenades: Notice that when you shoot a player multiple times with a
plasma pistol it slows him down. Toss a frag grenade or a plasma grenade by him
and let loose the shots. It'll slow him down so he can't escape the grenade.