Be a Hotwire, and rush the Obelisk yourself. Your Med Tank will make it, still draw fire off the Hummer crew, and let you get the points for destroying the Obelisk. Don't ever sit in the open drawing fire, at least try to do some damage yourself. In Under, the Obelisk is easy to toast if you go at it with a Med Tank. Prox mine your Tank when you get out! More than once I've done this after jumping out, and heard 'boink' from inside the building BTW- It takes only 1 Flame Tank to toast the AGT in Under.