I need help in Final Fantasy X Against the Ale Bhed Machine gun.

How do you beat it?

Ugh, it's been forever... >.<

I had the same trouble. Have all of your characters equipped with lightning-based stuff, because that's his weakness. If he's charging his weapon, have your party attack as fast as they can. (Haste may help.) If it's too late, summon an aeon so they can take the blow.

Focus all of your attention on the Negator at the beginning. When you've taken care of it, bring in Lulu to spam Thunder (Thundara if you have it) as much as possible. Since it will use Assault to counter magic, be perpared to heal Lulu often. If it's about to use Mana Beam, have Yuna bring in Ixion and use Shield. Then keep Ixion in spamming Thunder Magic. Even if it summons the Negator again, Ixion will remain on the field, though it can't use magic. Destroy it with Aerospark, and continue the attack. If it has 2000 health or less, use Thor's Hammer to finish it off.