The last boss of the Tri-Ace game for PSX, "Star Ocean 2." While being moderately difficult under most circumstances, a special sidequest could be done in the game to fight him "Limiter off" at the end, in which case he was hard enough to make Ruby Weapon, Kingdom Hearts' Sephiroth, and Penance all beg for mercy.
"You removed Indalecio's Limiter? You're not beating the game for a while..."
The last boss in Star Ocean 2
Man...Indalecio just kicked my ass.