Hmmmm, haven't fought them yet, but judging from the regular fight exorcist lady is useful (weak to ice + you can have all enemies with 0 atk and matk), Yolnaya with simple AOE nukes (followup 2) and maybe Lusica with the barrier skill and buffs?
Also make sure to remove anything that gives you fire element attacks
If the level is high enough, Dita should be fast enough to use her bind skill first (if you grind enough and teach her the "remove bind res. skill you can have the enemy bound on 4 of 5 turns; combining that with null def should give you a window to deal a lot of damage. I don't remember that mechagod's gimmick, if it had one.