After you learn how to press to block in the tutorial, you can now get this achievement at any time. You must be attacked a minimum of one time by the enemy for this to count, so if you can fight as few enemies as possible, do as much damage as possible on your first turn while leaving one alive. Let him attack and make sure when you block it, you see a Perfect below your PP. Finish off the last enemy and you'll get the achievement after the battle is complete. I found this easiest on a set of 2 elves in the neighborhood. You should easily be able to kill one on your first turn, let the second one attack you and make sure to block it (their attacks are pretty basic and predictable), finish off the second elf and the achievement will unlock after the battle.