I love Nature Form. Its attack is certainly useful, especially since most enemies can't seem to grasp the concept that thorns=pain.
Also, here's a fun one: Throw some briars onto a rock, bind it to yourself, turn back into nature, and swim around with that thing. It's like a giant flail of death 
that's a crazy attack

sort of have to be careful not to flail yourself when switching out. I've been toting a rock around for shield and fishing, so this just upgraded my rock, behehe. I got about 7-8 good seconds out of this attack using the keyboard switching technique mentioned above. launch the seed onto the rock; swap out to Naija; bind; swap out to nature. it's not too much more effective then just towing a rock around and there is lots of opportunity for Naija to prick herself on it, so not sure how often I'd actually use it, but very creative regardless

nature has some peculiar relations with some of the kelp forest denizens. the bouncy jellyfish that you pinball off, she dosn't seem to be as adversly affected by them. she can play pool with those bouncy spiky orange things, as well as those tentacly, invicible critters that are attracted to song.
Lailoken suggested somewhere else making a briar tent and hanging out in that, and have used that to good effect, also making briar walls on either side, or in a cave system having a stalactite and stalagmite system going. if you can cage a monster into a pen of briars that's pretty damning, bouncing off those will finish it off quick. you can advance through an area by going behind a briar, launching one, then going behind that, and setting up a very effective defensive enivornment.
a couple of monsters react a bit funny to the briars, those bouncing gremlin like creatures that take a few hits and turn red, they seem to damage the briars, and the mergogs bounce right off screen? except for mobs in open water, nature can really devestate, and even in a lot of those places there are mushrooms to plant them on. it's a bit of practice to figure out her lob, but after a bit you can predict fairly accuratly her range and the angle they'll pop out at.