Go into to his garage. Fix the electrical panel that operates the hoist. Step back towards the middle of the big door. There is an opening between two of the crates. Using your grenade launcher in first person targeting, send in a grenade. If the Thing turns its head toward you, then you have him targeted. Continue sending in grenades until he begins to burn. You can send in a few more if desired for extra fun. Run into the Thing's room and on the far wall from the big door is a switch. Throw the switch and watch as the Thing dies. You will probably take one hit from him.