On Act one scene two when you reach the bomb take out the guards then send your team to the crates with the yellow moving thing by the entrance. then go and stand behind the pillar furthest to the left and aim at the door on the second floor on the far right. it will say team 'move to door'. press it and your team mates should dissapear then reappear behind the turret. then if you press a on team 'Disarm bomb' the one of your teammates who defuses will appear by the bomb but the other one will be behind the turret still. so now when the enemy keeps spawning your teammate will be there to kill him as well as you so you can leave your character for a while but still get exp
. if anyone is having trouble with this send me a message on xbox live and i will invite you to a game and show you
oh and i forgot to add that to get the enemy to keep respawning is to throw a grenade against the door next to the bomb after the bomb is being defused
please send me a message if you need help