While I do agree with saving team, I don't think we understand difficulty spike in the same manner.
For me it's matter of relative difficulty difference between floors n and n+1. Let's pose a question: if I can barely clear floor n, can I clear n+1 ? Or another question: if I can clear floor n without problems, can I clear floor n+1 ? For me answer is no for both 11 and 13 floors. Comparing them is quite hard though, because without enough power they require vastly different strategies.

I have another question that came to my mind after reading your post: does weakening mobs from some floor with off-element not invalidate element mission ?