I did it quite recently, I hope this helps (but I’ve heard it’s inconsistent, so take this for what it is):
I’m not sure about the ledge thingy, I’d heard about it so I kept it in mind, but I don’t remember if the last time we stood there for a bit or not. Probably not.
We split up so my friend got the chains on the right, I dashed to the others, this made it even quicker than breaking them all by conc dashing myself.
The achievement unlocked right in front of the bridge of shadows for me. I used 1 purple potion with decanter (handmaiden) in the whole event, drank it right after the rat ogre appeared. My friend was way behind, he didn’t even make it inside in time, so that shouldn’t be a problem.
I’ve seen only another video of a guy clearing the event recently - for research - for him it unlocked there as well.