Alright, follow these steps:
- teach musharna charge beam
- lead sharpedo + musharna
- turn 1: use destiny bond and calm mind -> sharpedo dies and takes down toxapex
- bring in charizard, she brings in drapion
- turn 2: fire pledge the drapion and use x-defend on musharna -> charizard dies to stealth rock from nido and drapion knock offs into musharna
- bring in toxicroak
- turn 3: toxicroak uses full restore on musharna, musharna psychic the nidoqueen -> nido dies
- turn 4: toxicroak super potions the musharna, musharna psychics the salazzle
- turn 5: toxicroak super potions musharna, musharna psychics venusaur -> toxicroak and venusaur die
- bring in golem
- turn 6: explode with golem and psychic dragalge
- bring in whatever
- turn 7: super potion the musharna and charge beam the drapion.
I was able to replicate this three times in a row, so it seems to be a robust way of beating her even with damage rolls etc.