Love playing showdown but all I've gotten recently is bad rng. But I can give you a great smeargle in anything goes, carried me up to 1600. lol
Here's how to play: Ability: Moody, give it max speed
Set: Spore, Substitute, King's Shield, Leech Seed (w/ Leftovers or Focus Sash depending on level of risk)
This set outspeeds any slow threat, most Primal Groudon and Primal Kyogre sets, and Xerneas. You can also get lucky with some Pokemon if you get enough defense with Moody.
Eventually your stats will accumulate so much your opponent will forfeit.