If you're on your toes and have some half decent damage mitigation, it's not a huge deal to beat him solo.
Yes I'm sure there are plenty of people out there that has done it, the point of this video was to show you can do it so they change the boss cuz atm you can glitch his aoe making him an easy 800 stone farm
OP when you title the post "solo imperial sewers boss" it makes it look like you are showing off an achievement rather than displaying concern over a bug - i.e. the boss getting stuck and not firing his formidable AOE. Perhaps consider a re-title if that's possible.