If you are talking about fighting sin when you are on the ship, then it is pretty easy. Make sure you keep Tidus, Auron, and Rikku on the initial start of the battle. Because sin will be close to you then. Both Rikuu and Tidus can tell the Uncle when to move the ship back and forth. However, once you move back then switch your players around like, LuLu and Waka who can cast a faraway spell or throw the ball far to hit sin. You can use Yuna for curing and everything else.
if you are talking about the sin at the very end of the game (also the last battle in the game and with all your allies) make sure someone has the movie zombie. you need zombie because sin always uses the move curaga it give sin 9,999 health and you have no aoens. every time you die auto-life comes. use zombie on sin and when he is affected he will still use curaga and lose 9,999 health. make sure to get more mana to each player so he can absorb it than use curaga. one by one he kills him self.
If your talking about that sin on the boat, just use lulu and wakka and yuna to kill sin. then when you get all of those close ranged enemy (i don't remember where they come out) changed to auron tidus and rikku. make rikku use steal on the robots to do a k.o.
STEP1:Get Anima
STEP2:Grand Summon him
STEP3:Use his overdrive
STEP4:Get Yuna's overdrive again
STEP5:Use it to Grand Summon Anima again
STEP6:Use Anima's overdrive again
Last Step:Use Pain
Extra Step:Watch the scene
I managed to figure it out by summoning Anima and using his overdrive twice. Well, his overdrive was set pre-battle, and it would help to have Yuna's own overdrive qued in so that she can summon the 'megasummon' or whatever her overdrive is for bringing Anima out into the field.
you fight it
Yes.
72 hours
go to the village that you got arc
You get it after you beat Hecatoncheir in the Cave of Shadows.
to beat him use chaos sharo, then when your GB is filled up,use it
You don't beat him. You offer him dried meat, and he'll help you.
weakness is holy
you unlock a alternitive ending
Have Kain jump on her when she turns into a tornado
to beat baigan you need to cast reflect on cecil always attack with yang cecil palom
Use ye fu**ing water spells