Hmm...
This demon is harder than I anticipated. But the Knight dude down the stairs seems even worse... not sure What I wanna do right now.
Save the knight for later.
The demon is teaching you how to find opportunities, and using your environment.
There is a ladder to your right once you walk through the fog, kill the 2 undead up there shooting at you.
Then you can go down, trigger his spawn, and either....
A few strategies I have, read if you don't care and want to try them out.
Lock on and walk back, keep close to him, not too close, but if he's running at you you're too far. When I say running, he's actually running.
Then when he swings you go in and attack a few times then walk back, and when you're nearing the end then roll under his legs or something, or just side step around him and repeat.
or!
Trigger him, run to the ladder and climb up then plunge attack on him, then run to the other side, then back to the ladder and climb up, repeat.
or!
Trigger him, run up the ladder, and wait for him to jump up there. That way you have a wider area to fight him with so you're not constricted.