I managed to do it, with some RNG. In particular, his Munna, Pidgeotto, and Noctowl were underleveled.
I leveled his Pidgeotto and gave it Feather Dance. Munna gets leveled until Hypnosis and around level 25 for some basic stats. And Noctowl boosted until Echoed Voice .
Whirlwind to fish for Crobat and blow away poison mist, soft reset if I didn't get Crobat. Feather Dance when I get Crobat.
When Pidgeotto dies, switch to Munna and Hypnosis. Psybeam until dead. Hypnosis next pokemon, switch to Noctowl, and set up Reflect. Spam Echo Voice. His Skrelp has Focus Sash, which annoyed me, but wasn't much of an issue. Munna and Growlithe died in the end, but whatevs~
Game.rxdata