At this time there are only two creatures that have qualified as boss’s for me.
Daemonhost - about 25% into the game, shortly after finding the forge techpriest.
Decimator - about 50% into the game, shortly around the time you find Klosterheim.
Took me three character to obtain (two for Crusader, two for Psyker, one for Assassin). I executed each one, not sure if that matters.
Hope this at least gives you hope it should work. Power level didn’t seem to apply as my assassin was low level due to speed running campaign.
All this is subject to change as this game is pretty broken at this stage IMHO.