I've been using Haxorus, Darmanitan and Carracosta. I just unlocked the Super Train, clear streak.
Haxorus has Expert Belt; moveset: Outrage, Swords Dance, Earthquake, Brick Break.
Darmanitan has Choice Scarf; moveset: Flare Blitz, Stone Edge, Earthquake, Hammer Arm.
Carracosta has Quick Claw: moveset: Surf, Aqua Jet, Stone Edge, Shell Smash.
Can't remember the natures... but they were all from my in-game team, so they aren't correctly EV-trained... Still, I managed to do the 21 battle streak, and I'm only battling in the subway to get some of the ev-boosting items...