Visions of Mana

Once you beat the main story, you will now be able to load your clear save file (it will have a crown on it) and proceed into Chapter 10, the post-game chapter. The boss needed for this achievement is at the end of this chapter. As the description says, you can change the difficulty to Hard for this achievement. Despite it saying "midway" you can actually do this directly before the fight (but not during the fight itself), so feel free to play the rest of the game's content on any difficulty you like.

My final party for this achievement was Val in his base Guard class (which allows him to equip any unlocked bonuses from any other class - I picked the best ones from his Duelist, Liege, and Berserker classes), Morley in his Nightblade class, and Careena in her Divine Fist class. You should have all the best weapons for them from the ancient ruins and post-game chapter bosses. You should make sure you turn in any corestones you have and get the Dark Shield+ ability seed and give that to Val with any defense oriented ability seeds. Give Careena any defense and MP boosting seeds and make sure she has the Revifier and Strenghten+ ability seeds. Make sure Morley learns Crystalline from his wood elemental plot, and has as many strength, damage, and critical boosting ability seeds as possible.

Before you initiate the fight, make sure you are controlling Morley and set Careena's strategy to Protection is Paramount so she'll just stay back, survive, and revive anyone who dies. Right away when the fight begins, press  and switch to Val's ring menu and have him use Dark Shield+ which gives everyone higher Dark elemental resistance. Also use Crystalline on yourself and Careena as Morley. From there, simply hold  with Morley to use his special attack, which teleports him to the enemy for a flurry of slashing attacks. Once you land back on the ground, hold  again, and repeat until the scary dragon dies.

You may need to use a few Cup of Wishes if Careena dies so she can continue reviving and buffing Morley and Val, and possibly a few Faerie Walnuts to top off everyone's MP so you can keep up Dark Shield+ with Val, Crystalline with Morley, and keep Careena reviving and buffing. Save your Class Strike (the  move) for when the dragon charges up a huge AOE attack that will cover the entire field, as this interrupts it and will let you avoid what would otherwise be an unavoidable attack. There's really not much to the fight outside of that! The video below has almost my exact strategy, except he didn't use Dark Shield+ which I think is extremely helpful. Good luck!