Invincible Grunt
From Halopedia, the Halo wiki
At the beginning of the level Sacred Icon there is a red armored Grunt by the large tube. Open the tube. When the grunt comes close Melee him into the tube, then push the red covenant cargo box in the tube and then jump in. You'll soon realize that the grunt can't die even with a Grenade.
NOTE: The Sputnik skull is recommended.
