A; I have two answers for this because you didn't mention which version of the game the question was directed towards. On the original (FFIII SNES and PSX) it's Kefka as we can only beat a ff game up until the final boss cause it'll just reset with us, again, fighting the end game boss). For the Gameboy advance version you can do ALL the boss battles over again in the 2nd optional dungeon, the Soul Shrine.

Q: If you happen to not individually fight every monster one area of the game it had to offer and cannot return there, where is the ONLY dungeon to have a CHANCE at 100% bestiary?