Very easy and unmissable achievement, but:
Some players, including me, couldn't get it to unlock in very rare cases. I tried everything there is to try: Hard resetting the console, reinstalling the game, deleting files from the console and download them again, nothing worked. I even waited a few years since release to see if it would pop by itself or if Blizzard would patch the achievement, which both didn't happen.
The only solution is to[b] completely delete the game and the save files from your console and the cloud. I know, extremely annoying, but If you really want those 100%, there's no other way that worked for me.
Important: There's an achievement that wants you to reach level 70 with 5 different classes, and the one with 500 bounties. So you might want to finish those first if you already progressed far enough.
I did this one last and had to delete all my characters, which was tough of course, but finally hitting 100% was worth it. Good luck!