This Achievement can also be unlocked in the post-game. After completing the game at least once, Rapple will expand his inventory of Alchemy Synthesis recipes, one of which is a purple Dragon Aspect called "Dragon Reverse":
If you do not have the needed crafting materials already, a "Chicken Heart" can be rarely stolen or dropped from Thunderbirds, which naturally spawn in Sharion Coast, and later across the Windia Grasslands during windy weather, as well as in the Grimoire dungeon, "Acqua Alta, Undersea Temple."
"Thyroxine" can be stolen or dropped from Blaze Froggers, which naturally spawn across the Ulde Desert during a heatwave, as well as in the Grimoire dungeon, "Lavalode, the Scorched Ruins."
Equipping the Dragon Reverse Aspect will revert Dragonshifting from the the un-berserkable Awakened Shining Dragon form into the beserkable Restrained Shining Dragon form.
Once this Aspect is crafted and equipped to Yuma, the process to unlock this Achievement is the same as Ontyy listed. You can drain Yuma's MP quickly by repeatedly spamming Forces such as "Dragon Fight" and "Dragon Defense" during combat.