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: * Arm it with a Plasma Rifle. * Open up the piston and push him off, down to the first level. * You can now melee it, and it will not flinch from attacks nor will show blood when fired at. You can throw grenades at it, but it will not die.