Three Invincible Grunts Glitch
From Halopedia, the Halo wiki
The Three Invincible Grunts Glitch is a glitch on the Halo: Reach level Exodus. By bypassing a barrier and getting to an upper floor early, the player can encounter several invincible Covenant enemies.
Get to the area where you meet up with the Bullfrogs. Battle through to the next door where they say: "Other side, other side! The entrance is on the other side!". As soon as you get the nav point, turn around and go back through the door you just came in (the purpose of this is to eliminate the barrier.) Turn left and you should see a hole in the wall with some struts in the shape of a triangle covering it. Jetpack up to the struts and land on them, let your jetpack recover, then jetpack to the floor right above you. On this floor there should be a Phantom, three Grunts, and one or two Sniper Jackals, all of which are invincible.
The Grunts are unable to be killed through any manner, whether shot, stuck, meleed, assassinated, or headshot. While Assassinating them will not kill them, it will cause them to freeze in place and lose their weapon. They can be headshot multiple times, though they may give points for each time they're "killed." The Grunts are not completely immune to damage, as their methane packs and masks can be meleed off. This may cause them to fly through the air like normal Grunts, but it still will not kill them.