Great Jahy Will Not Be Defeated!

Once she was the second most powerful being in all the Dark Realms – Jahy, the Demon Lord's right-hand woman. But then came the magical girl, and in no longer than it takes to shatter a mana crystal, the world was destroyed and Jahy flung down from her perch. Now she's stuck in a child's body, lives in a crappy run-down apartment, and scours the human world for enough pieces of the mana crystal, obviously.

Super sadly this is the only one of its kind