This boss might look hard but he isn't. It will attack first with demi which cannot kill either of your characters so don't worry a lot about potions unless your health is low enough for one of the normal sinscales to kill you or if two can kill you. Otherwise just keep attacking until Auron gets overdrive which should only be in one or two turns. His overdrive is called Bushido and the only kind of Bushido you have is called Dragon Fang. It should kill the weaker sinscales and from then on in you just use your overdrives until you beat it. He only has 2000 HP so it won't be hard at all.
