To get the Witchunter achievement, you need to beat the game with a difficulty of 30. Naturally, the game sets the difficulty to 0, so, to get a difficulty of 30, you need to disable all the starting relics, and set all the options below the relics to the max.
Now, how to do it. Remove all relics but 1: the party mask. What this relic does, is when you or the enemy move a piece, it turns into a completely different piece. So, if you're facing one of the witches, and they move their witch piece, it'll turn into something different! Keep in mind, your pieces change as well, so upgrade places will only ever be useful for upgrading your king into the mounted king (your king and the enemy's king doesn't change when moved).
So, I hope this guide helps you get 100% in Ouroboros King!