I would wait for Curaja and the Bubble Belts.

Curaja is a big-time group heal that can counter the constant sap. Bubble Belts give you the double HP which helps a great deal as well.

Also, if you have them, it seems that the auto-sap in that area is taken off by Esuna and Esunaga. Quaklain can reapply them. But I recall going through that fight thinking I hit some sort of beneficial game glitch.

As for general strategy. The flans that accompany him should really be dealt with first. They are weak to Fire so spam your best flame spell on them or use fire based weapons... Fomalhaut + Wyrmfire Shot is nice here. Once those flans are out of the way, the fight seemed much more manageble.

OH YEAH! And try to steal a High Arcana from him.... you will thank me later.