You will find the ArcTrooper halfway through the Tharsis Refinery level, Lower Refinery Complex checkpoint. You can't miss him. Same strategy as with the Heavy. But instead of shooting his head, shoot the barrels that go on the wire above you when the ArcTrooper is close to them. He will turn around exposing flammable containers on his back. Run right up at him and empty your mag in his back (this will cause him to explode and you will probably die to if your to close, but hey you got the trophy!).