Geist's final boss wasn't hard. The hard boss fight was the one right after the helicopter crash. One good thing about the Wii is that button mashing to get out of trouble has been replaced by remote shaking, which would have prevented the problem I had because I swapped the analog sticks. That alone would make Geist 300% better on the Wii.
I played Chrono Trigger so much, always using New Game+, that Crono on my save can solo Lavos. I don't recall his exact level, but I think it's over 80.
I can't really think of a "hardest" final boss. Of all the final bosses I've encountered, I can't think of any that were actually harder than all the other bosses in the game. There's usually some optional boss that's just ridiculous, the final boss isn't a challenge because I've just got so much health and firepower by the end of the game, or I find some cheesy tactic. Sometimes, I just have an inordinate amount of trouble with an earlier boss.