This achievement is for defeating the boss Bed of Chaos. He is a super simple boss to defeat. The achievement is called "Defeat Bed of Chaos".
Once you get passed the Chaos Queen and ring the bell, you will then need to fight the Ceaseless Discharge. He is easy as well. All you need to do is get the armor in front of him and he will start to attack. Sprint all the way back to the white light door, and then the Ceaseless Discharge will try to pound you with his fist. Attack his fist a few times and he should just fall off the edge and die.
After getting passed the Ceaseless Discharge you will then need to fight the Demon Firesage which is basically the Stray Demon (first boss you fought in the game) on fire. Defeat him and go further down. Eventually you will reach another white light door and will have to fight the Centipede Demon. When you are fighting him, make sure to cut off and kill his tail. Doing this will give you a ring which lets you take minimal damage from walking on lava, which you WILL need later on. I'm not sure if the ring will drop after you defeat it, but it will if you cut off its tail.
Once you get passed the Centipede Demon you will have to past a room filled with giant enemies. Taking them on isn't too hard but they can be a pain sometime. If you have a lot of arrows you get them to follow you near the entrance where they can't get through and just chuck arrows at them.
After this area there are a bunch of Demonic Statues, and they are super slow so you can just run passed them. Go up the stairs passed the the giant vine enemy, and the first left turn there is a white light you can traverse through. Traverse through it and slide down the ramp. The boss is basically a big tree. On the sides of her are two orbs. Stick to the walls and go to one orb and hit it. She will then start to disperse the floor so watch out. Luckily if you die and come back, you don't have to hit the orb again. Hit the other orb as well. You can actually just use an arrow to hit the orbs as well if you have good aiming. You don't have to actually go towards the orbs.
Once you do that go to the middle, and run towards the boss and the floor in front will then crumble and this is where you have to jump and land a tree. Then all you have to do is cut the branches in front of you to reach the center and attack her soul once to kill her.
Here is a video on how to beat the Bed of Chaos.