Have the following party order: Fran, Basche (party leader), Vaan, and Vossler. Equip a character with low magic (in this case Basche) with the Nethicite, as this makes magicks take half damage. If you have the Evil Slayer, it is very helpful. Buy a lot of Handkerchiefs (oil remover) from the NPC outside and some Phoenix Downs. Before entering the altar to Belias, have Fran cast Protect and Shell on everyone. Next, have Vaan do the healing and Basche do the hitting. Have Fran cast Water and Slow on Belias. When Belias is at 40% HP or less, he will cast Firaja (different from Firaga) on you. This creates an Oil status which makes fire hurt a lot more. Cure this with the Handkerchiefs, and cast Protect and Shell on Basche. Keep this up and eventually you can acquire him as your first Esper (summons).