Marilith the Fire Fiend, is one of the Four fiends in Final Fantasy, first seen in FF1..
Her attacks contain Fire 2 and ice 1 (not sure correct me if i'm wrong) Pre-hard mode when in the hard mod world she has Fire 3 and Ice 2 respectively. She can be summoned through a Dying Fire Crystal, or you can summon her through a Fire crystal that spawns in the Underworld.
Personally i would jump around her and run around underneath casting spells or using ranged weapons. Shes not nice in CQB (Close Quarter Combat) so melee weapons are not suggested unless you got the best of best armor on.