The actual run starts at approx 25:55 if you want to skip to that. (I have to post the full unedited video from the main menu all the way to the kill to count for the WR - so I apologies for all the "wasted" time in the video).

Another slight variation on the kill technique to get the boss dead the frame he spawns in. But in order to shave a few frames and get the WR - we need to wait nearly 24mins once in game; this allows the the wind direction and wind intensity changes to give us about a 3% run speed increase in the direction of the boss.

This results in about a 300 millisecond saving - giving us the WR by a few frames!

I think a sub 15 second WR kill is possible with a different seed where we dont need to go around a stone - I'll keep searching and see if I can find one...

Seed used: GCmhQgH8hd