From what we can assume, Kain can not harm Kil'Jaeden. Although I am basing this on Archimonde's endurance as Kil'Jaeden has no set such. Although we all know that Archimonde and Kil'Jaeden differ in a large varity of ways. We need to let Blizzard have Kil'Jaeden engage in more combat before we can actually use him in a 'versus' and debate such properly.