Seth is a "must have" character for this fight.It would be great to have Ailment Resistance on all Immortals (learned from Angel Guard), and Absorb Damage skills. With these this boss encounter will be a piece of cake. Use 3 Combo and Forceus/Leveler on him and he'll be dead in no time. Standard attacks are very weak against this boss. It is worth considering leveling your mages (preferably immortals so they can make use of angel guards skill that protects them against all status ailments). Mack can also be useful in this fight by casting Minda/Mindus on your attacking mages. Another point to consider is that items are unavailable in this fight.
After battle you will get Gravity Rave and Gravity Ring combo for Seth (ultimate weapon apart from the white ignis but arguably it is better to get the gravity rave for Seth as you cannot have bothKaim and Seth's ultimate weapons). You will also get Aneira's Plume accessory that gives you Absorb Wind skill.