Walpurgisnacht is a particularly tough enemy from the Madoka Magica collab. This is because her falling building attack can destroy lots of cats at once and slow down any survivors. Her area of effect is tremendously wide as well. Also keep in mind that she is not a Floating enemy despite being in the air.
Surprisingly, this enemy becomes very easy to beat if you have unlocked both Li'l Madoka and Li'l Homura and use them in battle. The Cat Combo called My Very Best Friend involving these cats will further improve the effect of their Witch Killer ability by a massive factor. You can use also any of the Uber Rares from the collaboration should have have received them; see here for a list of units who have the ability. Without Witch Killers, the battle becomes more challenging, as weak or low-leveled cats will die in the process before being able to deal significant damage.
Alternatively, you can use Stone Cat boosted with combos to slowly drain its health, but be aware that in higher magnifications, Stone cat can and eventually will be one-shotted due to the Critical damage, and using it alone takes a long time to win. Still, Stone Cat can work wonders with stalling Walpurgisnacht while other units deal damage. Even if you don't have Stone Cat, you can use talentedJiangshi Cat and speed combos. Keep in mind this only works with a M speed combo and maybe lvl 8-10 or lvl 10 with a Sm speed combo. You also need Slow Resist to a good level so that it can move forward as soon as the knockback animation finishes.
She is tied with Homulilly for the second-most abilities out of all enemies, with Awakened Eva Unit-13 being in first place. All of these units are collab-exclusive units.
Walpurgisnacht is also the name of a German holiday. Walpurgis is a celebration commemorating Saint Walpurga starting on April 30 and ending on the 1st of May, while 'Nacht' means 'night' in German. Walpurgis is commonly associated with religion and celebrations and, like many things associated with religions, also with witchcraft.