Have you ever considered beating the final boss before doing a lot of the optional bosses? By simple virtue of the fact that they're optional and can be done at any time you can always wait. I never beat all the optional bosses before finishing the story, usually because they're harder than the final boss, and I'd rather finish the story as soon as I can.This means I get a fairly challenging final boss (as challenging as I can expect from an FF anyway), and still go and fight optional bosses if I choose to after the fact.