Extractor, 4k HP, weakness to Thunder. If you are referring to Sinspawn Ammes, then Dark Attack, and when the monster clears Darkness, use it again, use potions as necessary. Don't bother with the side fiends, not even to take a turn to kill one, because they are simply called, or if you miss, you waste a turn, and Ammes will simply call them all back again, because of their Suicide attack.

Otherwise...
Haste with Tidus. Simply attack, and if Wakka has his Overdrive ready, then use it, and hope you line up three Thunders. Extractor is immune to all status ailments that you can use thus far. Hurry and save Yuna from this malicious enemy.

If you want to be absolutely sure of yourself, then equip both Tidus and/or Wakka with a Thunderstrike weapon, but it is rather unnecessary. If Extractor prepares to use Depth Charges, then heal with Potions and High Potions, as necessary, or attack, if you have Sensor weapons equipped. Brotherhood for Tidus and any Strength plus 10%+ weapon for Wakka is recommended.

There should be no need to level up past 20 TOTAL Sphere levels for this battle.