Well firstly you can't injure the last boss till you visit the 4 corners of the dark world and fight a boss in each corner. After then you can take him on easily.

Anyways to go in with, if you got full onion equipment then I would see just how good your onion knights are, I hear thier supposed to be much stronger then the ninjas, if so go onion kids, Or maybe a combo of ninjas and onion knights or whatever.

Also for your sage, cure 4, life 2, cure 3, life. Black magic and summons are near useless in the last battle as he only has one attack doing damage to your entire party and most magic effects aren't worth the waste of MP and use of a turn in battle.