Invincible Grunt
From Halopedia, the Halo wiki
At the begining 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 then jump in. you'll soon realize that the grunt can't die even with a grenade.
NOTE: The spuknit skull is recomended.
