On the Smashing Tree level, run across the brook at the very beginning and you will find mushrooms. Then run up the side of the river until your Stealth Indicator goes yellow or red ( usually just yellow.) A short minifilm will play in which an evil tree sprinkles yellow stuff over Merry, Pippin and Sam, then gameplay resumes. When the tree tries to smash you, don't attack its trunk or it will kill Merry and Pippin. Attack the limbs when they try to smash you. The limbs will draw back with a weird noise. Note: Don't attack the limbs vigorously, or you'll lose health. Hit them one time with your walking stick, and then run.
Repeat this until another minifilm appears.