This trophy is called "Oops, My bad!" and it requires the user to attack an allied unit with an area-of-effect attack.
This can be earned during the Tutorial. At the start of the tutorial, move Arthur to the wolf in front of him and attack the wolf as directed. End Arthur's turn until it's Jessica, the Mages turn. Jessica is directed to use the Lightning spell in the tutorial. Activate the spell and target the Volga Wolf past Arthur and unleash the attack to hit Arthur and the wolf, unlocking the trophy.