Hmm.... I coulda sworn I was attacking second this round, something last night made me think I might be needing to go first.

Losing my miiiiiiiind, tralala!

Alrighty Neptune, he's in the water. Summon up a Whirlpool to try and trap him there.

If you successfully trap him, then make a nice wave with Surf and crush him. If he gets away, use Icy Wind to chill his wings and (hopefully) either slow him down or make him land.

And finally, put up your own Substitute, of a medium size.

Whirlpool ~ Surf / Icy Wind ~ Substitute