I don't remember the progression through this hold very well, but there's an NPC behind the green door in this room and you have to clear it to get something from them.
My process for clearing this room was, basically:
Click here to view the secret text
×I grabbed the speed potion, then ran for the upper right, killing stuff along the way as much as I could. After a while, I had a chance to slip past the enemies, after which the rest was simple.
This did feel fairly random, though, so I'm not sure if there's an easier approach.