Has anyone managed to kill it? Can it be killed?
Yes to both, I believe.
It’s not supposed to be killable, but I believe there was some trick or bug that allowed people to kill it at one point. I do not know if it still works.
It just replenishes all of its hitpoints at “death”.
It replenishes at about 20% health or so. I think the trick was simply to be able to do that much damage in a single hit.
The trick was to create the game with a level 1 character, then enter with a max level one and kill it. HP apparently depended on the creators level or somesuch.