Warlock - summon or revive a creature (maybe a fallen, starting with a Shank, evolving to a Dreg and ending with Randal the Vandal) to help shoot stuff or be a target to get attention away from you.
Titan - summon a solar machine gun, or a Hammer of Dawn from Gears of War type of weapon
Hunter - some type of freezing the enemies, like a trap or something that would stop them from moving for a couple of seconds