I find this set to be the most effective for him:
Hawlucha (M) @ Sitrus Berry
Ability: Unburden
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Acrobatics
- High Jump Kick
- Swords Dance
- Substitute
I've pulled off so many sweeps with this set. Sub is handy for Thundurus and also allows you to set up against Ferrothorn without being leech seeded. Sitrus so you don't get stalled out by sand. As long as they don't have Skarmory@Rocky Helmet, Zapdos, or Unaware this guy always puts in work.
I dunno how well it might work in OU because of Sand, but in UU I ran 28 HP EVs which brought me to 304 (which were leftover from aiming to outspeed base 100s), and that allowed me to get the boost after two Subs or one Sub + 2 SR switchins (I even won a game once thanks to that, my Lucha had been forced out by a Klefki and I could revenge an Outraging Haxorus because of that second SR hit). In this case, since you kinda want to outspeed up to Garchomp, it could be recommended to run 30 HP IVs (so 296) to not waste so many Attack EVs. That's divisible by 4 and 8, but not by 16 (unlike 304), meaning that Sand could potentially screw your set up. Otherwise you have to run 232 Attack EVs with 28 HP to still outspeed Garchomp, so pick your poison I guess.