You will get this when you're exploring the Imperial Palace after you've been made 1/1000th of your size by the Lucky Mallet.
Make sure that you have used a Mirror to save your progress recently. The achievements in this game are very temperamental and may not unlock when they should. If you do not have a save and you continue on after this achievement pops, you will need to play through the entire game again, up to this point.
There will be a point where you have to Power Slash a large flask down from the ceiling. Hop inside it, and then use your Galestorm ability to blow the mist out of the bottle. You'll then jump out, and meet Kasugami, and learn the Veil of Mist. Once you regain control, the achievement should pop.
If it doesn't, reload your save file, and try again.