
just have one of your mates go opposing team, use private chat on live, have him come to your base/you to his, stab,respawn,stab,respawn...etc
for engineer online :both of you in same team, your mate engineer class, you take a humvee drive to the edge of the map were its going out of bounds, drive out take some damage, reverse back,your mate patches you up and repeat....
for assault/spec op....online:
once again, your mate opposing team, private chat, just go to his base, find a nice place WAY round the back where no one is likly to bump into you, and kill,spawn,repeat...