Posted the same video a few weeks ago, but with wrong information: The bosses after Felmyst does only spawn if Felmyst dies.
So here we go again. Nothing wrong with the glitch tbh.
It's necessary to have a mammoth, and if you want to get under KJ's spawning point, you need slowfall too. Noggenfogger is usually doing it.
The video:
Hope you could use it :]