Dancing Tesla
Denchura@Petaya Berry
Timid; 6 HP / 252 SpA / 252 speed
Thunder
Butterfly Dance
Substitute
Bug Buzz
I haven't found all the counters, but off the top of my head, only Rotom Fire, regular rotom, Goruggo (ghost/ground), Gliscor, other Denchura, and blissey can really take the set.
After 1 Butterfly dance, and with 31 in Speed, SpA and SpD evs, his stats hit
SpA - 439.5 (586 if you factor petaya in)
SpD - 234
Speed - 519
Pretty much, if you can set up 2 Dances and the petaya, you can sweep just about anything. Also, IIRC, anything that has whirldwind/roar can be hit with neutral damage.
At first I thought about Energy Ball, but as I continued to look into it, his STAB moves and stat-up moves can more than make up for the resisted damage.