In this movie you will see how to enter Kil'jaedens chamber before killing any of the other bosses but you wont be able to spawn him before you kill Felmyst Im afraid.

I dont know if this way is known or not, if it is please forgive me =)

Music:
Metallica - mama said

Enjoy!

edit: Be sure to check out my other movie on how to get to Cthun.