Me and Twizted made a few discoveries while playing on Exodus.
We tried to resurrect a Civilian that we killed (that may be the reason why it wouldn't work) and the dead marine you see when the dropship full of brutes flies up next to you. It didn't work, and the grenade explosions don't seem to be as good as ODSTs, even with cowbell on.
Then later, we skipped a part of Exodus by flying up into the air, and then landing on a higher place. There, I jacked a phantom (entered turret) but it wouldn't leave, so I told Twizted to kill the rest of the covenant.
He did a grand job, but then these three grunts came out. They were invincible. I saw him shoot it 3 times in the face with a DMR, and the grunt birthday part effect happened each time, but the grunt would not die. That alone is a pretty cool discovery.
So I told him to go to the NAV point, so the phantom would leave. He did this, and then the phantom flew into the air as the death countdown started, flew me really far away, and then I died. Later I jacked another phantom but the damn kill barrier stopped me again.
At the end, where the falcons shows up, we managed to make one go under the platform. I got in the turret, and then he got teleported into the turret of the other one. The other falcon starts hovering in the air, and then Twizted randomly dies. My falcon would not move. Then I ended the game.
I will capture the invincible grunts and post it on youtube.