Diababa
Diababa is the boss of the Forest Temple, the first dungeon in The Legend of Zelda: Twilight Princess. It is a giant three-headed Deku Baba, corrupted by Twilight, that has taken over the temple. Once it has been defeated, Link receives the first Fused Shadow as well as the first full Heart Container.
Diababa has 2 stages, the fight against his hands and fighting his head. To defeat Diababa you must use the boomerang on the Bombling and on one of the giant Deku Baba's (Diababa's Hands). The Deku Baba should sink into the poisonous water. Repeat this on the other giant Deku Baba to defeat the first stage.
Once Diababa's full body appears in a cut scene you will find that the log the Bomblings were in were destroyed when Diababa fully appeared. Use your boomerang on Diababa to find it does no damage. After a while, Ook the Baboon will appear in another cutscene and will swing around on a rope above Diababa's head. The good thing about Ook's appearence is he will be holding onto a Bombling. Target the Bombling and Diababa's face with the boomerang to stun Diababa. Once he is stunned slash at the eye on his tongue do some damage, Rinse and repeat. When prompted use the ending blow on Diababa's Eye tongue to finish the battle.