Video of glitch fix/secret exploit.
There is some misconception surrounding this and No Stone Unturned. This achievement only applies to the SECRET areas (aka false walls that you can enter) in the game. According to the TA tracker, there are 43 of these in total. This makes sense because I unlocked the 50% achievement after 21 secret areas had registered.
Key points:
1. You only have to enter each secret area to get credit. If there's a collectible item like an energy ball or something inside you don't have to get the item for it to register.
2. SAVE before you enter each secret area! If it doesn't register, reset your XB1 and reload a save prior to getting the secret area. You can only know for sure if it registered if your achievement % increases, so this is easiest for those on a fresh gamertag with the app snapped.
3. There are 43 of these secret areas. Some ARE inside the dungeons, so they ARE missible, which will lock you out of the achievement.
4. There is currently an exploit to get extra credit for a secret area. Credit to WhiterValkyrie and to Krazie X360A for expanding on it. Video of this demonstrated is above.
1) Create a backup save (Don't think it matters when or where)
2) Find an undiscovered hidden area, enter it and create a soul link
3) Exit game, load backup save
4) Create a new soul link on the backup file somewhere
5) Exit game, load main save (should get credit for another secret)
6) Rekindle the soul link on the main save
7) Exit game, load backup save, simply rekindle the soul link you already created
8) Then just jump back and forth from main and backup save, making sure to rekindle the flame each time.