SPOILER
Right First things thirst he's hard too beet but at the right level he's too easy,
get about level 50-60 and go to the floating place in tehella
one of the building u can go behind to a giant rock sticking out of the ground examin it, it tell u u need to prove ur self with certain element stones/rings
equip Ruby, Garnet, Opal and Aquamarine onto ur 4 fighting characters, he will then say u still need to prove urself
Right At the end of the game when u arive back in welgia to fight yggdrasiel u'll find a Deris emblum when u get this go back to the floating island and re talk to the stone for the fight
He takes a beeting and as the bloke above says he has hard spells
keep raine healing, have gennis use spells,obviously u have to have sheena, but the last character make either lylod/zelos,kratos or presea
ps i'll have a decent huide to the devils arms and how to get gennis to learn meteor rain soon