Gokudou-Kun Manyuuki s01e18
Titre original : The True Identity of Me, I'll be a Monkey's Uncle!
Using Gokudo (in the pop star persona of Gon-Gon) as bait, the gang manages to infiltrate the secret monkey encampment ruled by Gokuu (or, as is currently the case, Niari - who has been unable to escape the watchful eye of Gokuu's gorilla-esque, shrewish wife). After a close call, Niari is rescued and things are beginning to look up for our heroes. (Niari even takes the opportunity to magically alter his body from monkey to man-form.)
Unfortunately, there's been an imposter lurking in the ranks, and it soon becomes apparent that what they had taken for Gokuu is in fact Sanzo the Monk in a copy of Gokudo's body (the real one sequestered away at some undisclosed location). He's been watching the lot of them ever since they unintentionally brought destruction down upon Inaho (in episode 13), and has lured them all to this place to serve his own dastardly ends. A battle naturally ensues, but the gang is outclassed by Sanzo's trickery... not to mention a magical 'damage redirection' spell.