Video for verification:

This strategy is decent. The biggest risks are UC guards and beakthings. I'm happy about the time but not about the run itself - mouse got laggy and right-click + scroll malfunctions stressed me out, as you can see in a shop & while training stealth. Got lucky to execute with lower stealth here. Seems that there are some math magic with stealth-states and gurglers on the go.

Tips for challengers:

I chose Freedom Seekers start because of proximity to the King Gurgler and initial athletics of an Old Soldier (6th character).

Its crucial to assassinate King Gurgler before the sunrise because of stealth bonus you get in the dark.

Assassination bonus is useless due to multiple-clicks trick, which allows to make more hits per one arm swing. When you aim to KO King Gurgler - click / "C" his lower-body :D. It feels like player is harder to spot on the highest game speed.

Total stealth with gear bonus needed for the elimination seems to be around 54 or little less + night bonus - to ensure consistent runs. So train accordingly.

There are different order to train needed skills. You can first train assassination on a Thief Boss (risk - UC guards) and then train stealth in the tower. If you train stealth first you'll be harder to spot by guards but Thief Boss will move more active so run might be worse as a result.

It seems that some fishmen & even King Gurglar itself spawn on the lower floors of the ruin and get up by foot. If you try to slower the loading of ruins via camera movement (aim for largest distance between ruins and camera) it may make sneaking past them easier, because they will be on the go. So you might need less than 54 stealth. There was 1 time out of 15-20 runs I could get King Gurglar on stairs, so it might be the key for faster runs - lower stealth requirements. And better routes, of course.