Garchomp is plenty fast to outspeed a lot of things, and with Sand Veil + Brightpowder, he won't be getting hit very often.
Substitute|Swords Dance|Dragon Claw/Outrage|Earthquake\Fire Blast would probably be the best set to abuse this. All you really have to do is bring him out on something you know will be slower, like a Ferrothorn for example. A Starmie came in and got some lucky hits in, you can always switch to Magnezone or your own Starmie to take whatever it would dish out.
Anyway,
Garchomp (F) @ BrightPowder
Trait: Sand Veil
EVs: 252 Atk / 4 SDef / 252 Spd
Jolly Nature (+Spd, -SAtk)
- Substitute
- Swords Dance
- Earthquake
- Dragon Claw
Or
Garchomp (F) @ BrightPowder
Trait: Sand Veil
EVs: 252 Atk / 4 SAtk / 252 Spd
Hasty Nature (+Spd, -Def)
- Substitute
- Swords Dance
- Fire Blast
- Outrage
But honestly, the first one works better from my experience.
One last thing, you might want to consider putting Rapid Spin over Toxic on Starmie. Someone may have said that already, but just making sure anyway. Other than that, it's a really good team. 9/10.