I used Nature Form, to great effect.
Go down in the very bottom of the room, and shoot a tree onto an urchin so that it closes off that area. Keep regrowing the tree, and throw some other trees nearby. The boss will just keep running into them, and won't be able to suck you in because you're blocked off. Sometimes, though, he'll manage to suck you through the tree; use Beast Form to swim away, then go back down and use Nature again. Be sure not to kill yourself on your own thorns if you're in another form.
As an added bonus, Nature form keeps the urchins from hurting you.
This is why I love Nature form the best, it's got so many tactical applications.