Vile and Violent Seeds are the best things to use. I stun him with a Stun Seed, eat a Violent Seed, and then back up all the way to the back and pelt him with Thorns or rocks until he got closer. Then, hit him with a Vile Seed. Then, you're pretty much free to use any attack on him which will cause massive damage.
Finished it? Goodness, I've played through every game, or at least every generation, Red Rescue Team, Time, and now Gates. And you're pretty well off. Your level is good enough to get through the rest of the game, pretty much.