Head up the ladder that appears after defeating the Dancer and turn left to find a Soul of a Weary Warrior and Cathedral Knight.
Take the lift and roll off on the second level before it reaches the bottom to collect the Estus Shard and then stand on the opposite side of the balcony to observe the area below. From here you can use ranged attacks to defeat the Pus of Man near the trees below and then drop onto the platform below for a Titanite Chunk .
The part of the ground covered in fog is where toxic build-up will occur, so rush toward the trees to get to higher ground to stop the build up.
On the opposite side of the building is a Ring of Sacrifice and in the center of the area is a platform with a Cathedral Knight. From the platform you should be able to use ranged attacks on the nearby Pus of Man, breaking line of sight and backing up to avoid melee combat.
You can then head under the platform to attack a bunch of Rotten Slugs and collect the Claw , Shadow Armor Set , Black Firebomb x2 , and Human Pine Resin .
Enter the archway on the side of the area and take the lift, rolling off halfway to reach a ledge with a Dragonscale Ring . Turn the corner and kill the Lothric Priest before you drop down to the ground and then take a left to return to the previous area.
There is another Pus of Man on the right near a set of stairs leading to an upper ledge, so use the environment as cover while you fire ranged attacks to safely deal with it before checking under the stairs for a Dark Gem .
You can now head up the stairs to find a Titanite Chunk and then check the opposite side of the area for Human Pine Resin x2 and a Titanite Chunk .
Head back up the stairs toward the back of the area to fight two more Cathedral Knights for a Magic Stoneplate Ring and then enter the boss area.