I was going to post Mega Audino as a check and counter to most of Garchomp's sets, but I can't seem to find a use for it other than that, so I'm just going to do a poke that's more traditional.
Mamoswine @ Focus Sash / Choice Scarf
Ability: Thick Fat
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Rock Tomb / Rock Slide
- Icicle Spear / Endeavor
- Earthquake
- Stealth Rock / Ice Shard / Icicle Crash
Alright, this is a pretty common check to Garchomp. It can just hit it Icicle Spear, since it's a OHKO, and it destroys Focus Sash variants as well. Rock Tomb is helpful to slow things down to finish off with an Earthquake, and it's a great move to check Charizard-Y and Talonflame, which is the reason why this Mamo could have a Focus Sash as an item. Scarfed Mamo takes out Charizard-Y with Rock Slide, but it doesn't check Talonflames who are at +2 already, so I prefer Sash Mamo. Icicle Spear takes out Garchomp right away, with no unreliability that Icicle Crash and Ice Shard give; Ice Shard gives Mamoswine a chance at OHKOing Garchomp, while Icicle Crash needs to be run with Ice Shard to take out Sashed Garchomp safely (Unless if it's Scarfed Mamo, it can just take it out with two Icicle Crashes). Stealth Rock is just great support for a team, but you can decide to run an attacking move if you don't need the support. Mega Garchomp is checked the same way as Garchomp is, just beware for the 2HKO Mamoswine might get on M-Garchomp with Icicle Spear if Scarfed, as Mega Garchomp has a chance to OHKO with a 252+ Outrage and a 252 Spa Draco Meteor.
Calcs:
Mamoswine:
252+ Atk Mamoswine Icicle Spear (2 hits) vs. 0 HP / 4 Def Garchomp: 200-240 (109.2 - 131.1%) -- guaranteed OHKO
252+ Atk Mamoswine Ice Shard vs. 0 HP / 4 Def Garchomp: 160-192 (87.4 - 104.9%) -- 25% chance to OHKO
252+ Atk Mamoswine Icicle Spear (2 hits) vs. 0 HP / 0 Def Mega Garchomp: 176-216 (96.1 - 118%) -- approx. 87.5% chance to OHKO
252+ Atk Mamoswine Ice Shard vs. 0 HP / 0 Def Mega Garchomp: 136-168 (74.3 - 91.8%) -- guaranteed 2HKO
Garchomp:
252 Atk Garchomp Outrage vs. 0 HP / 4 Def Mamoswine: 123-145 (66.4 - 78.3%) -- guaranteed 2HKO
252+ Atk Mega Garchomp Earthquake vs. 0 HP / 4 Def Mamoswine: 136-162 (73.5 - 87.5%) -- guaranteed 2HKO
252 SpA Mega Garchomp Draco Meteor vs. 0 HP / 0 SpD Mamoswine: 157-186 (84.8 - 100.5%) -- 6.3% chance to OHKO