I've tried that so many times it's not even funny.

This is what i do:
Use Rikku and Grenades with RIkku
Then Use Kimarhi for overdrive Seed Cannon
Then use Wakka for element wheels
Negator is down
countdown
3
Then i use Yuna to summon Iixon
2
Ixion uses overdrive thor's hammer
1
Crawler hit's mana beam
Ixion down
Yuna summons Ifrit
Countdown
3
Ifrit used hellfire
2
Crawler hits
Ifrit hits blk magic fire
1
Mana beam
Auron hit's crawler
so does kimahri
so does Rikku using Grenades
Crawler uses Gun attack
Potions used on my characters
crawler hits me
I use wakka to take out the Negator
Negator down
Use Lulu to cast spells on Crawler
Countdown
3
Use Auron to attack
2
Use Kimahri to attack
1
Crawler hits me with Mana beam
My entire team is dead.

I've tried everything that i can, but i still cant do it...this is insane...i need more help...please, thanks.