To unlock McCoy Oni you need to evole the mirage robe. The mirage robe is the main weapon of Babi-Onna.
To evole the robe you need to max it out and get the attractorb to max level, too.
Then just play 15 minutes on any map with McCoy Oni to get the achievement.
Have fun!
Thanks to The S bot 9000 for the comment:
You can also unlock him by using the spell "IonicoIonico" in the scrolls of morbane (secrets menu)