Invincible Grunt
From Halopedia, the Halo wiki
The Invincible Grunt glitch can be done at the beginning of the level Sacred Icon where there is a red Grunt Major cowering next to the Piston. To make it invincible, follow the steps below.
- Arm it with a Plasma Rifle.
- Push him off onto the first level.
- Melee it, throw a grenade at it, and shoot it. It won't die.
