Being far away from your enemy, sniping them (as in giving a warning shot), and watch them run in circles, jumping around, and dodging to avoid dying. I laugh my ass off everytime
I like to do that when were winning by like 15 kills on really open levels like burial mounds.
Heres a really funny prank I saw on youtube..
Get you and another person in a party and close it. Tell them to turn their handicap on severe and let them know what your doing. The plan is to invite a third person in and tell them you found a glitch that gives them juggernaut powers. I do this on lockout.. send the third person into the green room and tell them to crouch where the wall splits a little and run up the teleporter. Cause your handicap is on serve, 4 shots with a sniper just takes away their shield, while they can kill you with one headshot with the br.
Tell them it works in matchmaking too.. they get real pissed when they talk shit thinking they're gonna get 30 kills and end up with 10.