Ash's Muk Smothers Bellsprout in a Horrific Victory 

"The Fourth Round Rumble," an episode published by Indigo League

In the fourth round of the Indigo League tournament, Ash faced grass and bug trainer Jeanette.

Ash felt overconfident until Bellsprout vanquished Bulbasaur and Pikachu by evading with remarkable dexterity. Ash summons his Muk.

Muk can withstand numerous Bellsprout hits before crawling over it and suffocating it in poison ooze. 

The assault was a "Body Slam," but Muk merely slides on Bellsprout in a stinking wave. 

Given how the match transpired, it's easy to understand why Muk was benched. This is Ash's lone battle where Muk has prevailed.

Also See

Pikachu Obliterates a Gym Leader in One Shot