With Garchomp's great typing, good bulk, and powerful Attack stat, it manages to be a top threat in yet another metagame. Being one of the best Pokemon since its introduction in the DPP years, it really should come as no surprise that it's just as good in Doubles as it is anywhere else. The things holding this powerful Dragon-type back are having to rely on Dragon Claw as its main STAB and being slower than Latios and Latias. Garchomp definitely shouldn't be overlooked when team building, as it can easily rip apart unprepared teams.
The idea behind this set is fairly simple: do damage, hit hard, and hit fast. The two main moves are Garchomp's best STAB moves, one being a single target attack and the other being a spread move. Dragon Claw is the better choice in the first slot thanks to it being Garchomp's best Dragon-type attack outside of Outrage, which targets a random enemy in doubles and is thus unreliable. Dual Chop could also be used in this slot, due to it being a multi-hit move that allows Garchomp to break Substitutes and Focus Sashes.
There are several options for the third slot. Rock Slide brings the risk of having two spread moves, but it can hit Flying-types and Pokemon with Levitate harder than Garchomp would otherwise. Fire Blast is only used to hit Pokemon like Ferrothorn and Scizor that Garchomp wouldn't otherwise be able to touch. If Garchomp is paired with a Pokemon like Scrafty, Hitmontop, or Togekiss, Substitute can be used to gain free turns thanks to your partner, making Garchomp even more of a pain than it already is.
For the item, Yache Berry and Life Orb are the only two good options. Garchomp is incredibly weak to Ice-type moves, making Yache Berry a very attractive option that could buy an extra turn by staving off an OHKO. Life Orb gives Garchomp's already scary Attack stat a boost in power, making it an even more terrifying threat.Team Options & Additional Comments >>>
There isn't much else Garchomp can do, but it can run a few interesting moves that could surprise your opponent. Dragon Tail has higher priority than Trick Room does, so Dragon Tail Garchomp could work like Roar Latias and phaze your opponents before they even get the chance to set up Trick Room. Stone Edge is a single target move Garchomp could use that is stronger than Rock Slide, but it is less accurate and cannot make the opponent flinch. Outrage is a option, but you can't control it's target, and you also can't switch out, which is likely to be fatal in Doubles. A Swords Dance set can be attempted, but it's harder to pull off in Doubles. Lastly, Rest paired with a Chesto Berry can be used as a form of recovery for Garchomp.
Checks and Counters
Due to its glaring weakness to Ice-type moves, Cresselia, Mamoswine, Weavile, Choice Scarf Cloyster, and any Pokemon with an Ice-type attack are all good counters to Garchomp. Pokemon with Intimidate such as Landorus-T, Hitmontop, and Scrafty all give Garchomp a hard time by lowering its Attack stat. Landorus-T is notable because it also avoids Earthquake. Politoed, Ludicolo, Kingdra, Rotom-W, and other common rain Pokemon can be problematic for Garchomp too. Another very large problem Garchomp has is faster Dragon-types, such as Latios, Latias, Choice Scarf Hydreigon, and Choice Scarf Salamence. Trick Room is a problem for Garchomp due to its relatively high base Speed, so it should try stay away from Trick Room as much as it can.