Isn't it just completing Unknown Signal mode? That's not exactly a big ask. It's basically just a post-game free roam mode with 10 extra challenge rooms to clear. I played a little bit of it yesterday and managed to clear half of them fairly quickly.
Some of the chambers require you to fight harder versions of the bosses to unlock (I had already met the requirements to unlock the others), but with my almost fully upgraded gear I found them easier than the original fights...