I will confess to using certain exploits to defeat certain bosses in Demon's Souls, specifically:
-The False King (1-4)
-The Maneaters (3-2)
-The Old Hero (4-2)
-Old King Doran (1-1 Optional Boss)
In my defense, it was Demon's Souls.
In Fallout 3, there are 5 Super Mutant Behemoths, but you only ever get a fair 1-on-1 with two of them. The BoS more or less takes care of the on outside GNR, and the one in the Congressional Building can't reach you if you stay in the hallway. The last one is inside an electrified cage at a slavers camp, and if can manage to take out the slavers without damaging the power supply, you can just just fire at him from a safe distance.
Lastly, there's EVERYONE in Soul Calibur 4's Arcade mode, which, as Yatzhee also discovered, could be easily dispatch by throw-spamming.