Go With Adamant Nature
Even without Jolly Nature, you can outspeed some Dragon Types like Mega Sceptile. Garchomp with Jolly Nature plus Choice Scarf will outspeed Adamant Nature Garchomp. If you're up against Choice Scarf Garchomp you could switch out Garchomp for a Steelix that can Mega Evolve since Garchomp would probably use Outrage. If you somehow find yourself up against a Clefable that has a high defense, Poison Jab from Garchomp will be a guaranteed 2HKO with Adamant Nature. With Jolly Nature, it will be 99.6% 2HKO. It's not bad, but there is still a slight chance of a second Poison Jab not finishing off Clefable. You should try out this moveset:
Garchomp @ Choice Scarf
Ability: Rough Skin
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Earthquake
- Outrage
- Poison Jab
- Stone Edge
Sand Stream Tyranitar and a Steelix that can Mega Evolve might be some good teammates for this set, as it can activate Mega Steelix's Sand Force.