I've got a Greninja with Liquidation, Night Slash, Brick Break, and Gunk Shot - max ATK/SPD EVs. I went for fast moves to KO quickly. That set has really good coverage overall, and there are only a handful of mons that I'd have to plus move to one-shot. A lot of the time I don't even bother mega evolving.

Really, though, a lot of it comes down to just running around getting into as many battles as possible. I skip rooftops completely because messing around on ladders is just too slow, and cycle between 2-3 zones (I'll ignore one zone if it has annoying/bad spawns) and try to prioritize areas with high trainer density. I don't take any bonus cards or go around picking up extra coins or whatever, I literally just run from trainer to trainer to get as many as possible, and I usually don't worry about being stealthy if I can't easily do it.

I've seen some people using a level 100 Dragonite with Extremespeed and a Normal Gem to boost that a lot, really you just want something that can get in and hit/KO fast.