Besides a couple dozen retarded gimmick boss battles, I'd say perhaps... the Dahaka in Prince of Persia: The Warrior Within? It's freakishly difficult to beat and probably requires a very complex strategy to defeat, coupled with lightning reflexes... normally.
I, however, found out after ten or so frustrating defeats that if you knock it off the platform and jump-chop it in the head over and over, it doesn't seem to climb up, and you can keep on hitting it for a small but steady amount of damage until the bugger gives up and dies. This also prevents it from switching attack patterns, invoking more swears and pointless repetitions of the first parts of the fight as an attack I could never have predicted wipes me out after the pest steals all my time-rewinding sand...
I found it funny that I could win what was probably intended to be an epic-tastically difficult fight without spending a day or so figuring out how.