The Gyrados was his centerpiece. Pachi had Volt Absorb for safe switching on common Electric threats and built bulky to take abnormal amounts of damage.
The Pachi shined as a crippler (like Rotom), and Volt Absorb/Follow Me keeps Gyrados safe while Dragon Dancing. He didn't Mega until Earthquake was no longer a real threat, and Rock Slide could be dodged by going Mega form. Nuzzle is another rare move, providing a Taunt-proof paralysis. If he loses Gyrados, that set falls apart, but it's really hard to beat Gyrados with great support like that. Ironically, Conkeldur isn't the right MU with a Mega Gyrados, Machamp would've been much better imo, but Conkeldur is much more common with his bulk.
The real reason I think he'll be gone before long is OR/AS will change the meta again in 3 months. Hopefully without the dumb Kalos only clause so I can use Arcy again. q.q
Oh, it also keeps Garchomp safe with Follow Me against his checks. Honestly, a team with Garchomp and Mega Gyrados is hard for me to call off-meta. He outplayed everyone, Pachirisu just happened to have a skill set perfect for his prediction game.