When playing Fallout 3, at the point in the game where you first go to GNR to meet Three Dog, you will encounter a Behemoth Super Mutant. The easiest way to take it down is with the Fat Man mini nuke launcher that is available on the corpse of a nearby BoS initiate, located next to the fountain in the courtyard. I'd suggest using V.A.T.S, as the launcher's range is pitiful, and nukes are quite expensive (50-150 caps).