Wait, he was... defeated!? It's been a while, but I can't remember if that's happened before.
He has proven to be particularly susceptible to poison and such material (remember when he was hallucinating). So it isn't surprising that someone for once realized that the simplest way to deal with the unkillable Monk was to simply use some sort of poison/toxic substance.
Unlikely that he can be killed with posion, but I wouldn't be surprised if he was the type that would get drunk by single alcoholic drink (he is a ditz after all)
Are you all right!?Masteeeeeeeerrrr!!!That demon stung Master with her tail!!Ugh...Something's not right! There's no way you could be down in just one attack...!Hahah, gotcha!Ugh!Master??