You need to beat/catch a munna from a mirage spot so you can DexNav chain it- Once you encounter it enough, they might have a hidden ability. So stock up on pokeballs to catch a bunch of munna with! Once you get the hidden ability one, evolve it into a musharna and then you will have a Hidden ability musharna! :D
Another way would be looking for hidden ability musharna/munna on the GTS!