To counter Firefox, you will need 100 MDEF and 100% Neutral and Fire resistances. You will also need to use Full Chemical Protection.
She changes her element to Holy at ~25% HP. You can counter this with the Dark (Shadow), Undead, and Neutral elements.
- Gibbet Card (+5 MDEF) [Headgear]
- Goat Card (+5 MDEF) [Armor]
- Seeker Card (+10 MDEF) [Shield]
- Megalith Card (+7 MDEF) [Footgear]