Unlock the final Co-op challenge. Have player one choose Snake and player two choose any character. When you start the battle, which you will fight everyone with no stop, have Snake stand under the middle platform and keep pressing Up + A to do his Upward Grenade Launch move. This works because as he keeps doing this, the enemies keep spawning on the middle platform. The bombs will blast them out with one shot. Player two can attack the ones that player one missed.