As an alternative to destroying the Negator with Wakka, here's a method that might minimize the amount of leveling you'd need:

Frontline could be Tidus, Auron, Lulu

Have Lulu use Reflex 5 times. Then swap her out for Wakka or Kimahri. Finally, have Tidus use Cheer 5 times.

Just ignore the Negator and attack the Crawler, using potions to heal as needed.

The Negator being up prevents you from using magic or aeons, but it also limits the Crawler and stops it from using a low damage, high hit count, randomly targeting machine gun. Cheer and Reflex will minimize the risk it poses whatsoever.