My strategy for Upper Cathedral (once you get inside) is to stealth backstab the first one hunched over the Choir Set. Go to the left and down to the door that leads to the main area. Drop a trail of Shining Coins so you can find your way back to the doorway with ease and bait the three Wolves in the main hall to chase you back to the doorway. Kill them from the other side and head back in. Kill the fourth Wolf either in the open or drawing it back to the doorway.

Go to the right of where you enter the main room and wait behind the entrance to the corridor with the second Brainsucker. As soon as it passes the entrance jump it with the fastest attacks you have - anything with thrust and/or Fire Paper works best. Grab the Cosmic Eye Watcher badge.

Go to the corridor other side and kill the Wandering Madness if you have time - don't chase it into the room at the end where another Brainsucker lies in wait. Walk to the end so you can see it without it seeing you. Go in swinging so it can't react keep it going until it's dead.

Climb the ladder and wait on the other side of the entrance to the next corridor for the third Brainsucker to make it's way to you. Just as it reaches the the room you're in jump it. Collect the loot and open the shortcut so you don't have to fight the Wolves again if it goes pear shaped.

Now the tricky part. There are two awkwardly placed Brainsuckers and you need to kill them to get the key. You can use a Blue Elixir if you like but it's not essential. If you're in NG three or four Molotovs will dispatch the first one before it can get to you. Don't go out too far or the second one will attack - and you really don't want that. If it does, then beat feet out of there and come back after they re-set. The last BS can be killed with Molotov's from a distance far enough away that it's Paralysis mojo hits the stairs it's standing on - if you get too close it will attack. Bone Marrow Ash gunshots work well if you run out of Molotov's.