Ziggurat 2

Pretty random. As soon as you unlock the spell you can either pick it up in the game or (better) start your run with it when available (also random). There are 2 kinds of flying skull enemies in the game, you need the small, fast ones (not the big ones that spit fireballs). Kill one with the spell, done. The kill is easy, but having the combo of spell and the particular enemies spawning is random, might take a few tries. Obviously longer runs give a better chance of running into the enemy needed.