The rule is called “PokeSlots” apparently, where you match the same amount of Pokemon on your team as the gym leader.
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TRANSCRIPT:
Cutiefly: Well… here we are… Turffield Stadium.
Cutiefly: Are you two ready to get our first Gym Badge?
Corinth: Duh! I’ve been ready this whole time!
[CORINTH – Level 18]
Leeroy: Okay, uh… remind me why it’s just me and Cor again?
Corinth: Corinth!!!
Cutiefly: Because Milo only has two Pokemon on his team. 2 vs 2!
Leeroy: I guess…
[LEEROY – Level 20]
