You know, this a suggestion I'm not sure of, because my first thought was basically this plan, just be confident, but, uhm.
We could consider literally facepalming and saying "wow, I did not mean to deliver that to you that way."
Just don't be worked up about it. Just facepalm, call our own derp, then proceed with something more concrete than this stuff that's being suggested. I dunno... Oh, I do know. "Walpurgis -- that's the witch -- is much more powerful than the witches we usually deal with, but the city will be evacuated, and, well, we hold back, normally, a lot. There was this one time, Mami and I cut loose just a little for just a bit -- we built a forty-foot armored mecha with shoulder cannons and everything, and we just walked through the barrier in the conn pod. I'm not asking because I'm afraid of Walpurgis, although more help is always good. I'm asking primarily because the kind of person willing to help with something like this is also the kind of person I want to be able to support, and to be on good terms with. And, admittedly, secondarily, because Walpurgis doesn't hide in a barrier and so minimizing property damage will be difficult."
... Yeah, I'm more happy with this idea than the other votes, though it's largely because explicitly the mecha is going to be much more effective at calming doka, I think.
[X] Remain casual.
[X] Facepalm. Wow, did you just deliver that to her that way? Sorry, Madoka.
-[X] City-destroying, potentially, life-threatening, no. The city will be evacuated on the civilian end, and, well...
--[X] We hold back. A lot. Explain the mecha that we made with Mami the one time, including -- emphasizing -- the ease with which it allowed us to crush that barrier. That was just Mami and us cutting loose just a bit for just a little while.
-[X] More girls would help limit the property damage, though, which could be considerable, and... You quite like Mitakihara intact.
-[X] But we're also interested because the kind of person who would help with something like this is the kind of person we want to be in contact with, the kind of person we want to support.