Cult of the Lamb

To get this achievement, you have to have beaten all four Bishops and unlocked the Gateway door. You all need 20 followers in your cult as well.

If You Need 20 Followers

(This method currently works, I'm not sure if this will get patched at some point, but if you are late game (have beaten all four bishops and unlocked the Gateway) and have a good stash of money, you can use this method.)

At this point in the game, you should have the Spider on the right side who sells followers in their web.

1. Walk over to the Spider
2. Buy the follower
3. Enter the Gateway
4. Exit the Gateway
5. Walk over to the Spider
6. Buy the follower
7. Rinse and Repeat

The Fight

The fight starts with you picking one of three weapons and one of three curses to choose from (I chose the strongest weapon and a ranged attack) before battling The One Who Waits. Upon approaching The One Who Waits refusing to bow will start the fight.

Baal and Aym

• Then you will fight one of his followers Baal. Baal has a spear attack that comes up from the ground and will follow you around. This shouldn't be too difficult to avoid, as a red target will be on the ground to show you where the spears are gonna come up from. And they have an attack which is almost the same thing, but the spears are fixed in an X pattern.
• Upon beating Baal, you then have to fight Aym. Aym has basically the same moves as Baal, but also has a fireball ring move that they use as well.
• These two aren't too difficult of a fight, especially if you have done the no hit achievements for the Bishops. In my opinion their fight attacks are easier than some of the Witness and Bishop fight attacks.
• For the most part these battles are going to be spent dodge rolling until the Baal/Aym stops their attack. When they are done with their attack either use your curse or weapon to attack them. Both should drop some type of health after beating them.

The One Who Waits.

Stage 1)

This fight again is not too difficult and is mostly spent dodging, as well. The One Who Waits does borrow some moves from Baal and Aym. This fight is basically avoiding the spears, and the various fireball patterns.

Stage 2)

The One Who Waits will send out three eyes which will shoot rings of fire. Attack the eyes. When you've killed an eye, the the One Who Waits will send out rows and columns of fireballs, which you will have to dodge roll to avoid. After the third eye is destroyed, you truly beat The he One Who Waits.

You will then have the choice to spare them or murder them.

Note: I did this fight on easy and had the Temple fully upgraded. I'm not sure if that truly mattered though, since the max weapon damage I could select from was around 5.5. I also had the Fleece of the Glass Cannon selected, but when I when into battle I was wearing a different red fleece, so I'm not sure if the Fleece matters either.