Get BP in the Battle Maison

Mega Kangaskhan, due to its excellent ability. Run the following moveset:
- Power-Up Punch
- Return
- Earthquake
- Sucker Punch/Crunch

Lead with Power-Up Punch if you feel safe doing so, then finish with the other moves. Its ability breaks through Substitutes, Focus Sashes, and Sturdy, and doubles up your attack boost from Power-Up Punch. Have team support for things it can't quite handle, like Gengar (immune to all of Kangaskhan’s attacks except Sucker Punch/Crunch. If you chose Sucker Punch, Gengars found in the Maison spam Hypnosis so Sucker Punch will not reliably hit).

Kangaskhan can be found in the shadows in the first room of the Glittering Cave, but it’s super-rare.

Be warned, the AI cheats a fuckton, especially as you build up your streak. If your MegaKhan stops winning, try other exploitable sets such as

Durant - Truant (hordes in Terminus Cave)
- Entrainment

Dugtrio - Arena Trap (Route 13)
- Protect
- Hone Claws
- Earthquake
- Rock Slide/Stone Edge

Lead with Durant, use Entrainment. Switch for Dugtrio. Use Protect on the attacking turns, Hone Claws on the slacking turns until you’re ready to sweep. Not only will this make Dugtrio stronger, but it will help minimize accuracy hax used by the AI. This particular set is only enabled by the AI’s refusal to switch out.

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Another Alternative in Super Singles
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Starmie (Lead) im half asleep now but the item that boots dmg of super effective moves
Timid (natural cure)
252 SPA 252 Speed
-Surf
-Psychic
-Ice Beam
-Thunderbolt

Scizor leftovers
Adamant (technician)
252Atk 126Speed 126 Health
-Bullet Punch
-X-scizzor
-Brick Break
-Swords Dance

Garchomp Focus Sash
Jolly (sand Veil)
252Atk 252Speed
-Outrage
-Earthquake
-Fire Fang
-Swords Dance

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Other alternatives include any of the top teams from 

HERE

Although many of these are specifically geared towards B/W Battle Subway threats, they can work pretty well.