If you are having trouble unlocking this one keep some saves from around mid to end game. Once you've finished everything else you want to do in the game, locate the save game folder, and either disable cloud save or your internet. Copy the entire folder to a safe location, I just copied it to my desktop.
Save folder is usually located in c>users>username>appdata>local>DaedelicEntertainmentGmbH
Start Blackguards. Now go delete the whole DaedelicEntertainmentGmbH folder (making sure you have it copied elsewhere).
Try to load a game to make sure there are no saves.
Close the game.
Go back to the DaedelicEntertainment folder which should now be re-made, and find the folder for saves.
Copy only your save file that has the potions and poisons you want to use. Don't copy anything else like the caches etc.
Start the game, load this file, and the achievement should work now. Also fixes the Hand of Boron achievement.