To start a battle with a player, type !challenge @User – the other user will then have to type !accept to start the battle.
Use !fightp to attack, the battle is very similar to how you battle wild Pokemon.
When a Pokemon faints, you can choose to !cancel the battle and just have it as a 1v1 battle, or you can continue and make it a 6v6 Pokemon battle.
To continue after a Pokemon fainted, the user that his Pokemon fainted has to !use a different Pokemon, that Pokemon has to already be in his !selected list, you can add Pokemon to that list using !select but that has to be done before the battle began. After the user chose his new Pokemon the battle goes on and you basically repeat this until all 6 Pokemon of one of the players faint.