Hmm... I don't recall ever getting down to critical levels in that battle. Did you have your characters capped? His attacks did a shitload of damage at times, but it was easily mitigated with Combat Clinic, which was already selected before he did it.

I haven't done the end-game stuff, so I guess I'll give summons another try. I got bored with the game after the mediocre ending. Overall the game was way too easy.