If you’re up for a Pokémon Battle, you can choose “Single Battle” for classic 1 vs 1 battles or “Double Battles” for more strategic 2 vs 2 battles. Besides the battle mechanics themselves, these two modes function identically.
At the beginning, one of the players can set the battle rules.
No restrictions is exactly what it says on the tin. Pokémon will enter the battle at their actual Levels, and will have their increased stats from eating candies.
Normal rules is a bit more sane. All Pokémon are reduced to Level 50, and increased stats from candies are ignored. Besides that, there are no further restrictions. So Mewtwo is still fair game.
Once the rules are decided, you can choose to fight with the Pokémon in your party or choose them from your Box. Next, you’ll have 1 minute and 30 seconds to select your team members and order. At this point, you can press the Y button to check your opponent’s team.
Then it’s battle time! Battles are the same as in single-player, but the “Bag” options is gone, so no items can be used. Instead, there’s a “Run” option that you can use to forfeit the match. Also, you’ll get 20 mins on the clock, and you have 1 min and 30 seconds to decide each move.