For those of you like me who didn't get this achievement the first time through and you're unable to get the achievement to unlock, you will need to delete your save file. You'll need to delete it persistently: see this guide - https://support.xbox.com/en-US/games/game-setup/cloud-game-s...
- you'll need to select "delete everywhere", otherwise the game will pull down your broken save file each time you start it.
You will need to complete the Baba Yaga DLC in order to unlock score attack for the two Baba Yaga levels Return to the Wicked Vale
and Witch's Cave
- I confirmed just getting into the vale in the game won't open these areas for Score Attack. If you run through the game on the easiest difficulty and don't complete challenges or collectables, you will need about 4 to 5 hours to get to this point. Assuming that you haven't hit the same bug again, the achievement will unlock a few moments after the first score attack begins.
If you have a second xbox (XBox B) you could try pulling down your save file to that one and keep it offline while you complete this re-run. After you unlock the achievement perform the same "delete everywhere" command on the xbox you ran this second run through (XBox A), then turn it off. Xbox Cloud Save logic may say that Xbox B has a save file where the cloud doesn't and will resync it to the cloud. This is not confirmed, though.