AH! That's not a strategy to lure him, so I'm going to work. :'(
But as soon as I grow again the pair needed to launch DS, I'm gonna try that approach, even tho it kinda seems pretty basic. Probably I'd need to attack him with an horizontally swinging weapon (like the black knight huge sword?) to throw him down the bridge, because my loved halberd isn't really useful, pushing him back but still on the bridge.
As for the demon: I figured out I needed to go behind it, like immediately. But the wave is circular, so it hits you even there. You have to get under it, because the wave doesn't start "inside" the demon, but just outside it, so there's that safe place, indeed, in its butthole.
And that... that I found annoying. To figure out this strategy you need to get in what should basically be the most dangerous place of the whole fight (and IIRC the demon also has a tail attack, maybe not, but still in DS you are used to approach carefully tailed enemies), hoping to be precisely in the tiny safe spot, because if you miss the position of like 20cm you get hit by the wave, thus not being able to figure out the strategy anymore.