This will come naturally through the Story Mode, Battle Mode, or S.T.A.R. Lab Missions. Simply build up your meter fully and save your Super Move towards the end of the fight. When your opponent has around 20-30% low health left, activate the Super Move by pressing + in order to unleash a devastating attack which will kill them, end the match and unlock your achievement. This can also be done easily in a local match with a second controller. If you don't have a second controller, it can also be done fairly easily in a singles fight on very easy difficulty, or online with a friend.