#1 - April 12, 2012, 9:59 a.m.
When you enter the level and are in a hurry and manage to click to the stairs and skip the cut-scene quickly, you will move so far to the right, that the 4 monsters won't see you and won´t get attracted to you and therefore not leave Cain. This means they neither attack you or can be attacked / damaged by you.
In the end I was able to attract them again one by one after running to the closest edge of the balcony where you are in the video, then kill one - then rinse and repeat. After that the other skeletons spawn.
Best Feri