At the very end of the Thorns Castle level, there is a big troll that you have to hit him in the mouth with pummelweed.
Use Pummel weed to uppercut the troll. When he stands there and doesn't move, morph into Chilla and use the icecles to shoot at his neck as many times as possible. Keep on doing this until the Troll's health runs out.
[thanks Kameocoolio]