User blog comment:Koishi-Komeiji/Iroha & Yachiyo (Final Battle ver.)/@comment-32751919-20191015012428/@comment-24965192-20191015121520

Walpurgisnacht is extremely powerful after all. Madoka is supposed to be the only magical girl able to defeat it, but at the cost of her own life, which produces the witch Kriemhild Gretchen who is even stronger than Walpurgisnacht (mega oof).

I doubt the combined efforts of all the magical girls in Kamihama would still be enough to defeat Walpurgisnacht. It's the attack Farewell Grief that Yachiyo and Iroha both used while in that form that ended the witch without any casualties.