For Tartarus, he's just tough so you have to shoot him a lot while dodging his attacks. He'll eventually activate invincibility and once that runs out just keep shooting and he'll die like any other enemy (scavenge power weapons that spawn with the boss rounds). For the prophets, you need to disable their gravity thrones. I found using a ghost and strafing runs was the best approach.

Double Needler

i just wait out his overshield and toss a firebomb and dump all my ammo on him. as for the prophets i just use a spartan lazer or a heavy vehicles like tanks or guass hogs

Chopper beats literally everything

To defeat the Prophets, shoot at them until they die.

throw grenades at them until they die hyjack their thrones if you want

laser the prophets and spam shotgun and dmr on tartarus