Skeletal Avenger

This becomes an option after beating the final sorcerer boss in the fourth questing area (top left area of the quest select screen). After beating this, the credits will roll, which you can skip. You'll be taken back to the main hub where you can check stats and quests. If you move to the bottom left of the screen you will now be able to open a locked gate and head through a door. Inthe new room, head upwards to a Throne you can sit on. You have to increase the level to 2 and then confirm which will start your New Game+ and unlock the achievement.

Doing this will reset your quest map progress back to the first area and all enemy types can now show up on any quest level. Your quest counter towards your 50 quests completed will remain intact along with all of your other stats, upgrades, trinkets, and perks. The quests will just be a bit more difficult ow. You can always sit on the Throne again and reduce the difficulty back down to level 1 if you find it to be too difficult though.