Fiends have infested the airship so there's plenty of places to level up prior to this fight, as well as 2 save spheres to keep you alive and all magic'ed up. I found it extremely helpful just to be strong enough to withstand its attacks by having enough HP to allow yourself to recuperate after its attacks in addition to the attack strategies listed above. Just staying alive is key to be able to heal or revive when needed. Like I said there are a ton of battles to be fought on the airship prior to this battle, hence tons of opportunities to work your way through the sphere grid to earn more HP.