The Multiple Invincible Grunts glitch will only happen in the Halo: Combat Evolved level, Assault on the Control Room.
Walkthrough
Just before you get to the Hunters, activate a checkpoint. Then, go to the U-bend (The hall with no doors and lots of beams) and search for an Active Camo there. You should find it near the second to the last beam. Once you grab the camo, run as fast as you can towards the Hunters, without shooting at them or touching them. You could grab some ammo at the weapon caches. After that, run toward the next door. If you lose your active camo before you can run to the door, or if you get spotted by the Jackals near the door, just load your last checkpoint. When you enter, you should see two Jackals, three Grunt Minors and one Grunt Major. Don't mind them. Just go forward until you Load and the words If I Had A Superweapon... appear. Then run back and you'd notice the Grunts don't move. They just stand there and whatever you do (even meleeing them, shooting them or throwing grenades at them), they won't budge. Go back to the Loading area and the Grunts will die if you meleed them, shot them or threw grenades at them. The same situation goes for their Jackal allies.
What happens in this glitch is that when Halo makes you go through a Loading station, everything behind the Loading station will be included in the game's visuals, but will not be animated/will not move.