Say they are on Max Stats for Speed: Bayleef: 218, Shuckle: 108, Caterpie: 188 and Electrode: 378. The battle attacks would be in this order: Electrode, Bayleef, Caterpie, Shuckle.
You get to select the Pokémon you get to attack so say you wanted to use Vine Whip as Bayleef, you get to pick if Caterpie or Electrode get hit. However some attacks work on both target pokémon like Surf and Blizzard. It is not yet known if the damage gets halved or not, however it'll be said here when it's known
Multiplayer Battling
Luckily, since the GBA can have 4 Players...you can have 4 Player Battling. You each choose 3 Pokémon and use them. It's basically run the same way as a 2 on 2 battle...except you can only control one Pokémon instead of the two. It is one of the options when you decide to battle in the Pokémon Center. Here are a few example Pictures:
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