The difficulty spike: how would you address this situation?

I'm a little late to the idea party, however something I've wanted to do to that battle (after ramping up the difficulty) was add a couple of guest characters. You could add a spider (Narsh guard or Figaro guard) and lock their gear. Giving them a cover skill and a protect accessory without hurting the rest of the game. Or just a general tank low attack high defensive gear.

Other than adding a temporary guest or two, you could just re-average the party level to something high enough to beat it at your desired dificulty level. Personally I always made a point to level up during the split scenario to make that fight easier, so if the player was over leveled then do nothing, if under leveled, reaverage them to minimum level. Can't remember the best way to d9 that in code, and no its not the best answer story wise, but bumping the levels up a touch wouldn't be noticed by every player anyway. (Some/most but not all.)

Guess you could also make the enemies give extra exp so they'd level up during the fight (doesn't leveling do a full heal too?). Then, they could struggle through a few, heal, and be strong enough to finish the boss by the end.

Tis a thought. Good luck,