Here's a bulky Musharna set I've been trying out to counter Nidoqueen.
Musharna (M) @ Leftovers
EVs: 4 SDef / 252 HP / 252 Def
- Calm Mind
- Stored Power
- Signal Beam
Roles: Bulky Special Sweeper, Lategame Cleaner
What It Does: It primarily counters the notorious Nidoqueen, but also can put a dent on most of the RU metagame on its own. It should only really worry about Dark-types, and even then it doesn't worry all that much about them because of Signal Beam, its coverage move.
Good Teammates: Heal Bell support. Musharna has Heal Bell itself, but it does not afford the luxury of using Heal Bell over Signal Beam because it desperately needs the coverage move. A lead that sets up hazards such as Spikes and Stealth Rocks can also help guarantee OHKOs/2HKOs for Musharna, so that is something to consider.
Counters: A few Pokemon counter Musharna. Whimsicott can shut down the Calm Mind process and annoy it to oblivion through several means, although this is mostly done if Musharna is used early. Dark-types in general resist Musharna's main STAB of Stored Power, which is what prompts the usage of Signal Beam as a coverage move. Sucker Punch is not pretty on Musharna, although it can survive many unboosted ones from various Pokes. Escavalier also qualifies as a counter since it can OHKO Musharna with Megahorn.
Overall, the biggest counter to a Musharna is its owner, especially since people can play Musharna incorrectly, lending it susceptibility to Taunts as an example.
Additional Info: Very underrated Pokemon on the whole, and is once again a great counter to some of the big bulky offense Pokemon that centralize RU's metagame at the moment.