HP: 10000
MP: 800
ATK: 75
MATK: 40
DEF: 250
MDEF: 100
AGI: 25
LUCK: 15
Resists Fire
Weak to Light
Weak to Thunder
Resists Wind
Heat Up II
Attack
Fire Charge I
Fire Charge II
Fire Charge III
Firestorm
Windup I
Rampage II
Dragon firestorm is one of the hardest bosses in the game. His stats are deceptive as he is always fought on lava and as such has his defences doubled, and regenerates 3250 hp per turn. For ATK builds, a combo of brittle, heat up, lightning sword and rampage can let you kill him in a single turn with a mere 57 ATK (on nightmare). For MATK, large amounts of thunderbolts seem like the best bet.
Alternatively, you can remove collar to easily kill the dragon, as its hoard of gold is surely worth it.