I'm nominating this pyukumuku set as a defensive
counter to the given musharna set.
Pyukumuku @ Leftovers
Ability: Unaware
EVs: 248 HP / 192 Def / 68 SpD
Bold Nature
IVs: 0 Atk
- Rest
- Block
- Toxic
- Soak
The combination of block, toxic, and rest allow pyukumuku to trap and poison musharna while healing both damage and status received from synchronize. Unaware allows pyukumuku to ignore musharna's stat changes, making it one of very few pokemon who can stop it after it has used multiple calm minds. The EV spread allows pyukumuku to avoid being knocked out with four psychics from musharna while also avoiding a three-hit-KO from max speed 252 attack silvally multi-attack 99.8% of the time. Having Pyukumuku on your team will make setup worthless for opposing musharna and will either trap it or consistently force it to switch out, benefiting your team greatly.
A downside to this set is its passivity. It loses hard to taunt and trick and can be broken by manectric, mr. rime, and glaceon. However, pairing pyukumuku with offensive threats and double-switching into them will allow your team to gain momentum any time that musharna is on the field.
Only one calculation matters in this case, since all stat changes are ignored:
+6 8 SpA Musharna Psychic vs. 248 HP / 68 SpD Unaware Pyukumuku: 78-93 (24.9 - 29.7%) -- 0% chance to 4HKO after Leftovers recovery