I've completed several of these. Last night was on the Moon. Me an one other Guardian took him down, but he almost made it to extraction at that semi-circular amphitheatre near the Hellmouth. It was tough though. The most difficult part was keeing his shields from regenerating. We figured it out that if we each took turns hitting him with a grenade, then unloading our HMG;s at point blank range, we would wear hhim down relatively quickly. So we switched in and out like a tag team match. lol At the end, two simultaneous supers, one from Voidwalker (me) and Bladedancer (him) and he went down with style. :D