What we need to remember is that we need a mon who:
a) Can set up SR and preferably other hazards as well
b) Can tanks hits from Garchomp
c) Has enough power/tricks to beat Garchomp 1 vs 1
With this in mind I'd like to suggest 
Ferrothorn
One major disadvantage of the Garchomp set given is simple: no Fire Blast. Without Fire Blast, Garchomp has no way to beat Ferrothorn one on one, locked or not so. Thanks to Garchomp's high speed, Gyro Ball hits VERY HARD.
This is the set I want to suggest:
Ferrothorn@Leftovers
EVs: 40 Atk/252 Def/50 HP/166 SpD
IVs: 0 Spd
Relaxed Nature
- Spikes/Thunder Wave/Leech Seed(on discussion)
- Stealth Rock
- Gyro Ball
- Power Whip
Firstly, the EVs. A little changed from the Standard set on the smogon website, the 40 Atk EVs allow Ferro to guarantee a 2HKO after SR. 50 is in fact all it needs to avoid a 2HKO from LO Earthquake: it becomes a 3HKO. The rest of the EVs are to allow it to come in later on other mons. Thanks to this Ferro can set up SR as a lead with Garchomp and proceed to then 2HKO, LO recoil only speeding up the process. Alternatively, on the turn Chomp sets up SR Ferro can Leech Seed, allowing it to drain Garchomp and thus set up better(after LO recoil of a turn, leech seed damage of two turns, on the third turn and when Garchomp attacks for the second time, it very likely KOes itself with Life Orb, meaning Ferro gets SR guaranteed vs Chomp)
40 Atk Ferrothorn Gyro Ball (150 BP) vs. 4 HP / 0 Def Garchomp: 168-198 (46.92 - 55.3%) -- guaranteed 2HKO after Stealth Rock
252 Atk Life Orb Garchomp Earthquake vs. 50 HP / 252+ Def Ferrothorn: 126-149 (41.86 - 49.5%) -- guaranteed 3HKO after Stealth Rock\
If Garchomp locks itself into outrage, Ferro has even better times:
Outrage: 25.58 - 29.9%
Thanks to good recovery in  form of Leftovers and Leech Seed this set has the capability to beat Garchomp hands down.
Also, this mon can help deal with Rotom-W thanks to 40 Atk STAB Power Whip considering it has only one weakness.
Even if Team 2 selects a fire type mon for the last member, team 1 already has a fire type counter called Slowbro so it's not an issue, Ferro can set up SR and run.
Please comment and let's discuss before we vote.