Cult of the Lamb

I'm going to post a few quick tips for beating all of the bosses without taking damage.

First, which is something that I didn't realize you could do until after I had already got all of these achievements, turn down the difficulty to easy. I'm regretting not knowing this one, as I did all these achievements on extra hard and for your sanity don't do this.

Second, since you're going to do a flawless run of the boss, use the golden Fleece , as their is really no downside to using it since you won't be taking damage.

Third, I would recommend doing these achievements after beating the game and getting all of the tarot cards. The tarot cards that I went for are ones that would increase my damage on my weapons or curses. Specifically, I went for ones that made it cost less energy to cast and if you can get them the one where curse energy is refiled on entering a new room or the one where it slowly recharges.

In terms of weapons I used:
I like going for the axes they deal enough damage to kill enemies quickly but aren't as slow as the hammer making you entirely susceptible to damage. This is entirely up to preference though. Although the faster you can get though the fights, the less chances you have of getting hit. I like trying to find godly weapons for this reason.
As for curses:
I used curses that swiped and knocked back enemies as well as projectile attacks. I mainly saved my energy in case of emergency to knockback incoming attacks. Some people might want to play the range game and that's fine as well.

IMPORTANT
This achievement can be made trivial if you can get lucky enough. While looking for which path to take, look for a modifier that looks like a sword between two hearts. This will make all enemies health is halved but you also receive double damage. This includes the boss you are trying to do without taking damage. I would go through the floor try to rack up as much damage bonus from the golden fleece and then go to the statue and kill the boss really before they have a chance to think about the sins they've committed.

Also as a quick tip, if you can do the brainwashing ritual before you leave, if you care about your followers. You may be gone for a while.

There is a method that can make these Achievements easier.

First you will need to have unlocked the Golden Fleece by spending a Holy Talisman at the temple in your cult.

While wearing the Golden Fleece, your attack will increase by .1% for each enemy that you kill without taking damage. Run through whatever arena you need a boss in and kill as many enemies as you can without taking damage to increase your damage percentage. That might sound hard, but once your weapon percentage starts to get high, you’ll be one or two hitting enemies. You can see your damage percentage in the upper left corner of the screen.

imo the Golden Fleece method is best paired with the projectile based curses and the heavy hammer weapons. The hammers already do a lot of damage, so they will be even stronger when used with the Golden Fleece. If you start a run and don’t get the weapon you want, you can immediately escape by holding LB. If you don’t have the escape ability unlocked, you can just quit to the main menu. Load up your game and enter the arena again to refresh your weapon choices.

When you’ve got the right weapons and a high damage percentage, challenge the boss. It is possible to take out the bosses in just a few hits with this method.

EDIT As Scarovese#3367 points out below, the bosses can be replayed. You will need to destroy the Devotion Statues in the combat arenas. They will have a red X on them. Destroy them and a portal will open up that will allow you to travel to the boss fight.