From the Super Mario Wiki
Trunkle is a giant rock monster with a small tree on its head 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 mineral menace, and the bros must save her. Trunkle has the ability to throw boulders at the brothers, which have to be countered with Hammer strikes. When Trunkle's HP is reduced by half, he splits into four tiny Trunkles. One of them has a berry in its mouth, and Mario and Luigi have to keep an eye on that one while all four shuffle 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 continue whaling on Trunkle until he is finally defeated.
Trunkle's body has extremely high defense (though not as high as the head), but the most effective Bros. attacks against it are Knockback Bros. and Splash Bros.
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 and cannot be woken.
Names in Other Languages
|From Ganjō, meaning resistant.
||Pun on roca (Spanish for rock).
||Literally: Strongrock, pun on "roquefort", a French type of cheese.
||Pun on geröll (German for scree) and the name suffix -inger.
||Pun on pietra (Italian for stone) and the Italian name Pietro.