How do you defeat sin?

you mean final fantasy 10 first part? you have to defeat that sin many times wich ship are you talking about? if you mean when gusys go out on a ship when it's flying to enter into the sin, where sin has little sity on it's head... i don't exactly remember. first of all you need very strong aeons, all of them died and only that horse survived because sin uses thunders and the horse is a thunder aeon too and the hit increeses it's life try this. if sin goes to fire tactic and uses fires you have to use ifrit. nothing you can do with gravity and gravija, if you gather enaf chocobo woings and other stuff(see in aeon learning abylyties) you can teach your aeon to curaga. also teach aeons thayr own magic i mean if you use that thunder horse, teach him thandaga and use on himself to increese its health. same to other aeons.i finished that game 2years ago and don't remember what exactly i did.last sin is veeery disgasting. in the end of a game you have to fight your own aeons and defeat tham.