252Atk Choice Band Hustle Zweilous (+Atk) Crunch vs 240HP/252Def Musharna (+Def): 90% - 105% (390 - 458 HP). Guaranteed 2HKO. 36% chance to OHKO.
That's about the only thing in NU capable of OHKOing Musharna. This isn't helped by the fact that all super effective attacks against Musharna do not have a base power exceeding 80 (except Punishment, which requires Mushy to acquire a CM boost beforehand), as Megahorn users are in higher tiers.