(In my experience, this only allows you to get the achievement, it doesn't allow you to keep the levels)Here is a solution I found. Although it talks about PS4 and a imperfect method for Xbox One, I was able to get it to work in part using
and
while I was in the load menu. BUt in my experience, I wasn't able to get out of the menu after collecting the luminious visages. So I had to shatter an entire clump of these (i think i had 75 at the time, and the achievement unlocked. I then had to back out to the menu and when I returned to the game, I was back at what I was before doing this.... so for me, i wasn't able to get out of the glitch to get it to work.
I will also point out that I didn't want to chance my apololipic run, so i completed the ng+ run, and when it gave me the option to start another new game, i cancelled, which when I resumed brought me back to the point before the final fight.
(NOTE: the gameplay menu is wierd, and even changing settings can change and void the difficulty achievement, as per comments and my experience of my first run)
I had done 2 playthrus, one of normal, and then my new game plus on hardest difficulty and done both DLCs, collecting everything and fighting everything, and I was only level 67 when i got to the final battle.
So this achievement requires insane grinding, even if you don't use any of the visages you get. I used most of them from my first run, and saved all of my second run ones, but still without this glitch, i would have (probably wouldn't have) grinded for many many hours or did another NG++ run, which I didn't fancy.
The video is NOT by myself, so give the author credit on youtube please.