Well, we all know the door bugs at Scarlet Monastery.
(If not, read the buttom part - I do not take credit for it though - its old and overposted!).
Anyhow - When you are inside SM: Cathedral, and you really want to fight the boss', but don't have the key to open the large gate, well listen up then.
First you need to clear the left side of the door, because behind the door there is a lever you can pull if you stand with you back to the doors edge at the wall.
If you are lucky you will be able to mark the lever with your mouse, and not the door - and voila - right click!
And for those who dont know the door bugs at SM, then listen up (This is only if you dont have the Scarlet key btw!):
First you run to the doors, (From left to right) Graveyard (no door), Cathedral (door), Armory (door) & Libary (no door).
Now to get past one of those two doors, you will either need the key - or you will need to die near them.
When dead you can go past the doors, resurrect and go into the instance - simple.
Now - If this simply is a repost (very old then, since I tried searching for cathedral with no real results), then I do not take credits at all for this, if not - well cheers.
-Added pictures, and if anyone got trouble getting it to work, feel free to message me or go to Argent Dawn (EU) and whisper Zakua for more informations.