The quest you need to complete for this achievement is 'The Thong of Compassion'. The Evil Nun is found in the Sanctuary up in the north in the top right of the map just east of Strumfuquel. When you first reach the Sanctuary, you will be blocked by the Monk of Admittance. To be able to get past the Monk, you must be in critical condition (when you're in critical condition, hearts will blink on the screen, while you hear the sound of a heart beat.) After you get past the Monk, you'll walk up the stairs to see two monks arguing. Convince them to go swim otherwise during the fight with the Nun, they'll join in to defend her. Finally, speak with the monk next to the door. Make sure that he goes inside twice. After the second time, you can go in.
Although you can get the books before all that, I chose to get them afterwards. You'll need five red books to enter the basement, one of which is in the main Sanctuary room. Another one is west of TriCore-7 with the large Leprechaun Mr. TangleFangle. Kill him to get it. Finally the last three red books you'll need are in the Le Beaux Kuckelle library. After you've acquired all the books, return to the Sanctuary and listen to the five Tourist Monks around the base of the Sanctuary. They'll tell you the story of each God and therefore, you'll be able to have the dialog options that will make the Gods not hostile with you. After you've listened to them all, go back inside the Sanctuary and go to the basement.
The Nun is covered in five force fields. You must deactivate all of them by going to each altar. Once that is completed, go to the Nun and the statues will ask you questions. Answer them correctly and not only will they not fight you, but the Evil Nun will not have their powers. The answers to their questions are:
Statue 1: It could take nine years and still cannot reach the perfection of your nose.
Statue 2: I will give all that I have to the Downtrodden
Statue 3: It would make flowers bloom and birds sing
Statue 4: The last thing you ever lifted
Statue 5: You are not confusing me
If you do not listen to all of the Tourist Monks, these choices might not be available and the statue will be hostile with you for what ever choice you've chosen, so please make sure you listen to all FIVE Tourist Monks.
The Nun is very simple to defeat. When attacking her, use your block as much as possible in order to save health. If you run low on health at any time, use a potion immediately, and if you do not have a potion, try to run around her in circles, as you eat food. Try to use a combination of attacks as that will give you your justice attack faster. Use your justice attacks when possible and the Nun will be dead in no time.