The forest boss can be defeated with a two pronged attack.

1. Use Pummelweed to pop under one of the explosive shells lying about as
big-boss-tree-guy sends waves of barbed roots at you. Once inside, aim the shell at the
boss, and just as you're about to reach him, use the uppercut (Right Trigger) to toss the
shell in his face. It should explode on impact.
2.

Once you've nailed him with the explosive shell, he'll have a little tantrum and
start shaking bemused trolls out of his hair. Switch to Rubble and roll close to the
boss's face. When he opens his big mouth, use the Left Trigger to lob a rock inside. If
you find yourself bothered a bit too much by the trolls he shook loose, just use Rubble's
Gravel Blast ability to stun the surrounding trolls. That should afford you enough time to
lob the rock in his mouth in peace.