This can be unlocked at the beginning of the game, but you'll get it throughout the game anyways without any efforts.
According to posts from several people, there seem to be different ways to unlock the achievement, but I'll explain the quickest way to get it:
After a short introduction on practicing melee, range and magic attacks, you'll have your first battle.
- First, have both Tobias and Calvin get close enough to the enemies.
- Then, when it is your turn to attack, have Tobias (or Calvin) to perform a range attack using his bow on an enemy.
- Last, when it is your turn to attack again, have Calvin (or Tobias if the first attack was by Calvin) also perform a range attack using his bow on the same enemy.
- Now, automatically your first character will repeat the same attack and you should see "Team attack" appear on your screen, unlocking the achievement.