With a single blow Trophy

After your first battle in the game, choose to go to the gym and use character gems to boost up your fighters under development. Focus on the strength one and then return to the menu select. Choose to do a training battle and you will take on the team you just did, only this time you will be stronger. Wait until you get to the final opponent (Locke) who only has low HP, then select Salsito and cast Embers on him (should be by default unless you changed it around) as this will do should do more than enough damage to kill him in one blow (just make sure you don't use that attack on the second opponent at all so it's available for you right away for the battle with Locke). If you don't get it there, finish the fight and power yourself up again in the gym, then retry. Once you finish off an opponent in one blow, the trophy will pop.