From Halopedia, the Halo wiki
The Invincible Grunt glitch is a glitch that can only be done on the level Sacred Icon in Halo 2.
To find it, play the level Sacred Icon on any difficulty. At the beginning, there is a red Grunt Major cowering next to the piston. To make the Grunt invincible, follow the steps below:
AI Super Jump • District loading glitch • Drive an AA Wraith • Engineer glitch •