SPOILER

Ok i´ll explain you then when you say you have defeated all of the bosses in Ultimecias castle you´re not correct...
There´s still an optional boss called Omega Weapon it´s the most powerfull boss in the game...but i´ll let you find out by yourself.

Just make sure you safe your game before doing this trust me.

So in order to fight Omega Weapon you need to form two parties i advise you to choose a party of your stronger characters i always choose Zell, Squall and Selphie trust me Zell is essential with is Armaggedon fist limit break.Now put that party in the green ring just outside the chappel where you see the purple cloud.
Then change to the other team and go to the room with the green ring just before the room where you fought Trauma and you´ll see a bell that you´ll have to puch then a timer will start quickly change to the other team outside the chappel and round inside you will now see a statue of the Omega Weapon run to it before the timer runs to zero and fight it!

Good luck you´ll need it and don´t be mad if it kills your party it happened to almost everyone.

If you can´t beat it i have a litle strategy but i think you should try on your own first.