Invincible Grunt
From Halopedia, the Halo wiki
The Invincible Grunt is a glitched Grunt in Halo 2's Campaign level Sacred Icon.
To find it, start the level from the beginning, where you will find a Grunt Major cowering next to the piston. To make the Grunt invincible, simply arm it with a Plasma Rifle, push it off onto the first level, melee it, throw a grenade at it, and shoot it.Template:Fact Alternatively, you can let the Grunt activate the piston, and then throw a grenade onto it for the test.
Note that killing any Constructors will disable the glitch.