First I would highly recommend that you have completed the main story. The setup we will be using will force you to leave your cult to their own devices for at most three days depending on how lucky you are.

If you have the fasting ritual perform that before you leave on a trip to perfect a bishop. If you do not have that ritual you will be under a bit more of a time crunch but it is still possible. Additionally performing the brainwashing ritual will help keep your cult together in your absence.

You will need to additionally unlock every tarot card that involves curse buffs as the fleece we will be using for this strategy is the Fleece of the Glass Cannon. Technically the Golden Fleece is better, but it has a higher skill cap. The strategy I will describe is more luck dependent but can easily be replicated by people of all skill levels, but if you feel confident with your skill you should be fine to just use the Golden Fleece.

Again we will be using the Fleece of the Glass Cannon for this strategy. Also you may want to turn the difficulty down to easy temporarily if you're having trouble. I've been able to replicate this strategy on Very Hard, but it feels a bit more luck dependent on that setting.

Throughout the run you want to prioritize tarot cards and weapon swaps. For tarot cards you will want anything that buffs curses. The big ones are the cards that reduce the Fervor cost of casting Curses, buffs to Curse damage, and slowly replenishing your Fervor over time.

The Curse we are looking to equip is Hounds of Fate, and in the highest possible level we can get our hands on, but keep in mind that a low level Hounds of Fate is better than a high level of any other Curse. We are looking for Hounds of Fate for three reasons:

1. It fires multiple projectiles which scale independently off of the Curse buffs you gain from both your Fleece and any cards you are able to pick up.
2. It has homing so you cannot miss very easily. While this homing is not as good for regular combat rooms the massive hit box of all the Bishops makes it very hard for these projectiles to miss.
3. Since this attack doesn't need to be aimed you can mash the Curse button to quickly fire it and swarm the Bishop.

While I'm sure there are other spells that can pull this off this is the least risky one I've found.

Now that we have everything we need let's talk about the actual fights. First; how do we even fight these guys after the main story is over?

If you didn't know, after killing a Bishop all the enemies in that area will get a buff and you'll be able to loop that dungeon up to three times. In addition to these changes you are also be able to break the devotion statues of the Bishops and doing so will open a portal to take you to a re-fight of each of the Bishops.

It is important to know that when you fight the Bishops your run will end and you'll be sent back to your Cult. You only have one chance to perfect these fights per visit. Only enter the fights if you feel like you're as ready as you can get.

Make sure you enter the fight with full Fervor. As soon as the fight starts hug the bottom of the arena and mash the Curse button as fast as you can. If you got enough Curse buffs the boss should be dead by the time you run out of Fervor. If you didn't prepare enough and the boss is still alive you have two options. If you got the card that slowly refills your Fervor you can focus on dodging until you have a decent amount of Fervor to nuke the boss again, but if not you are going to have to take some risks and attack head on. Quick weapons like daggers work well as you can get a hit and dash out of the way quickly to grab some Fervor. Killing adds is risky but also gives a good amount of Fervor if you feel like you can pull it off.

This strategy will likely take a few tries to get right, but with enough effort it should be possible for anyone to achieve this. Have fun and enjoy your 100% completion!