>>19715771252 Atk Mamoswine Ice Shard vs. 4 HP / 0 Def Garchomp: 276-328 (77 - 91.6%) -- guaranteed 2HKO
This is a terrible Mamoswine set btw because it has no LO/Band and is not Adamant.
Sashed Mamo will also OHKO with Icicle Crash
So yeah, Mamoswine pretty much OHKOes Garchomp.
It can also switch in:
252 Atk Garchomp Outrage vs. 4 HP / 0 Def Mamoswine: 237-279 (65.4 - 77%) -- guaranteed 2HKO after Stealth Rock
Neither will all mamoswine carry that little bulk. For instance,
252 Atk Garchomp Outrage vs. 48 HP / 0 Def Mamoswine: 237-279 (63.5 - 74.7%) -- guaranteed 2HKO after Stealth Rock
48 HP EVs are needed to barely survive an Outrage AFTER Stealth Rock 100% of the time and OHKO back with Ice Shard.
Yep, Mamo is a pretty good counter to Garchomp alright.