Well this place sure is creepy- she's used to the heat from fighting in the middle-east, but this sure does look like Hell, doesn't it? And those arms don't seem to be taking much damage from either of their attacks! Not that they intend to stop of course.

Once the creepy singing starts, they'll both get the idea to sing something else to counter the voice- and for lack of thinking of a more catchy song in the moment, they'll go with changing up the lyrics instead.

Hijikata starts by using his summer power to turn the fire green and harmless, before using his D4DJ mini-isekai experience to start them off, singing with his whole heart;

Not haughtiness, but bravery
Sent forth these two to come and see
What lies enclosed in glacier's heart
You can't take us down!

Mukuro meanwhile had poetry lessons with Monika in Avalon, so takes a deep breath and joins in with all the embarrassment she had when Junko made her sing that one time;

Here, now us knights have come
to bring down steel and magic and guns-
This day we will not fall.
Our skill shall end you all.