What are you playing? c'mon, tell me punk!

[Jermungandr]

It's been a long time since I played Pikmin 2, but if I recall correctly there is an easy way to deal with the water wraith without risking your pikmin. Just ignore all the treasure the first time through the dungeon, and head straight to the bottom floor. If you run straight to the exit on each floor fast enough he won't be able to keep up with you. Then after beating him on the final floor, you can revisit the dungeon and he won't appear, letting you explore at your leisure.

That's how I remember doing it at least. It's been years since I've played the game so I'm not a hundred percent sure on this.