That's not unusual for an Megaten game. Hell in Digital Devil Saga, I had my guys nullify everything except Almighty and almost had a full team wipe to one of the optional bosses because he started the battle with a Mind Charged Almighty Attack.
Sometimes you're just unlucky and these games tend to be very unforgiving. You'll just have to suck it up and try again. Especially if it was only one time.