Trunkle

Trunkle is a giant tree monster that inhabits Teehee Valley. Mario and Luigi run into him during the events of Mario & Luigi: Superstar Saga.

When escorting Princess Peach to Little Fungitown, she is attacked by the arboreal menace, and the Marios must save her. Trunkle had the ability to throw boulders at the brothers, which had to be countered with Hammer strikes. When Trunkle's HP was reduced by half, he split into four tiny Trunkles. One of them had a berry in its mouth, and Mario and Luigi had to keep an eye on that one while all four shuffled around. However, if Mario and Luigi attack Trunkle's "tree" on top of his head, they can defeat him without him splitting into four tiny Trunkles. His "tree" has much lower HP and much higher defense. The brothers continued whaling on Trunkle until he finally was defeated.

Interestingly enough, if the brothers follow the path to Little Fungitown before Princess Peach comes to the Beanbean Kingdom, Trunkle is still blocking the path, but is sleeping.

The easyest way to beat him from my opinion: Use a bunch of hammer-based Bros. Attacks on his little tree.