If your talking about the sinspawn gui then i think I can help you. When its head starts moving hit it with wakka. Take out the arms. It does less damage without them. Then take out the head. After that just keep hitting the body. If Lulu has second level magic then use it. Keep your health up and keep hitting the body.
You have to fight it again after but Seymour is on your team. And it has less health the second time. Hope that helps.