Instructions
Place Tidus, Wakka and Lulu in your party. Tidus will keep everyone up to speed, while Wakka and Lulu can be relied upon for long-distance attacks. Since Evrae can pull itself away from the ship and prevent close-range attacks, their ability to strike from a distance is essential.
Dispel "Haste" if Evrae casts it. A "Hasted" Evrae can execute several swoops to destroy your party before you can even heal yourself.
Cast "Dark" with Wakka to blind Evrae. When it regains its vision, cast "Dark" again. This will make Evrae's physical attacks nearly useless, as he will miss your characters most of the time. Still, keep them healed if they're injured. He can still get a lucky strike in.
Cast Thundaga or (if she has it) Flare with Lulu each turn. If she runs out of mana, have her drink an elixir and get right back to casting spells. After getting hit enough, her Overdrive meter will be full. Have her use her Fury ability in conjunction with Flare or Thundaga for major damage.
Command the ship to approach Evrae if he flies away. This way, Tidus can still attack.