Hello! My first topic here, if this has been posted before, just ignore it.
This occured when we ran Karazhan a couple of weeks ago, and I haven't read about any nerf so far.
Anyway, simply go Moroes, do the full fight, and when he is at about 5% or so, stop all damage and make sure that he us fully doted, if you manage to time the kill when he does his vanish, he wont count as an official kill, however, you will be able to loot him.
Doing this will lock the Opera Event untill he is killed properly, but all you have to do is to wait for a soft reset (2 hours) and he will respawn, this should be repeatable as many times as you want to, and when you would like to advance, simply kill him in the *normal* way.