- 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.
- If you kill any Constructors, the glitch will not work.
- This glitch can also be done by letting the Grunt activate the piston and then throwing a grenade at it. 
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