One of the very few moments I'll agree with Apex. Sharingan sees through Mu's invisibility - established and proven. The question is, though, can Danzo activate his Sharingan with Izanagi on? He never resorted this with MS Sasuke, as Obito said his Chakra would be lowered a lot more. But then again, even if he resorted this and saw Mu's invisibility, none of his arsenal have the range to attack him on air. Not Baku can guarantee when before Mu is swallowed, Jinton atomises it.
I'll go with Mu, high difficulty.