This can be unlocked during the final fight with Melfice which happens on Grail Mountain. Overall this isn't that hard to do you just have to make sure you aim to do it. Level up Flying Tenseiken and buff your party members and de-buff Melfice which you can do. As long as no one else damages Melfice you'll get the achievement after the fight.
When I went for this achievement I just had Ryudo use the Flying Tenseiken over and over again only targeting Melfice and using a SP recovery item with another party member if my SP got low and rotated healing, buffing and de-buffing with my other party members.