From the main menu, choose endurance and change the difficulty to Insane using the Right Stick
. Then hit Continue and at some point while playing it should unlock.
This did not unlock for me the first time, but did successfully the second (I also Quit the game and powered down the console in between the two attempts).
When this unlocks, if you don't already have it you will also pop the achievement "Nirvana" (playing a game of Endurance, any difficulty level).
(From the setup menu, after choosing Endurance, you should be able to combine this achievement with "Nirvana No Aimz" by toggling off the aim assist before starting, but that achievement does not appear to be unlocking for anyone, myself included.)
EDIT: The Devs have said on Twitter that this achievement should actually unlock on the
Hard difficulty level,
not Insane (which is actually higher).