Agent Horrigan, the FEV mutated secret service agent final boss of Fallout 2.
The fight was great, there where so many ways to kick his ass...
-Talk a squad of Power Armor wearing, Plasma Rifle toting soldiers into defecting and helping you fight Horrigan to get to the exit before the Posedion Oil Rig blew sky high.
-If you killed President Baxter earlier, you could use his key-card to over-ride the rig's security systems, initiate an anti-insurgency security protocol, causing all of the nearby auto-turrets to target Horrigan.
-My personal favorite, run up to him and keep using alcohol (Gamma Gulp Beer, Rotgut, etc.) on him. Each one lowered his Perception skill by one point, after about 6-7, Horrigan couldn't shoot the blind side of a barn from 10 paces. So at this point, you could finish him off at your leisure.