It's been a long time since i last played this part of the game but i'll try to give you some tips.
1.First of all level up your characters and don't rush into the game.The next battle is a lot more harder...
2.It's a machina so you must hit it with thunder.First equip your characters weapons with thunder spells,especially Wakka's so you can hit the negator.
3.When the negator is destroyed let Lulu hit it with an overdrive of thunder.(It's good to have your character and Aeons overdrives ready before battle).
4.Let Yuna summon Ixion and hit it with Thors Hammer.
5.Use haste to some of your characters.

I hope this helps,good luck.