When my character was unlocked, I was able to continue the story with +600 difficulty setting, At the end of the story, the last boss didn't go over 1300 or something close, can't remember because my other characters are stuck for a month now.

So the only way to kill a boss at PR 1500+ is only possible in warzone, but unfortunatly, as it is supposed to count toward the perk, well as many other things it does not !

Same for the trophy : kill 5 bosses : well the bosses from warzone don't count towards the trophy ^^

For that, as many of us did, we created a new character, rushed to the first boss battle (around 40-45 min of gmeplay), kill him, then rince and repeat.

If you try to go on with the story, you'll hit the other carshing bug, and there is a boss battle with a crusader teaming up with you (sorry can't remember the names, I'm not playing for more than a month now), if you're not careful, he will deliver the killing blow and then : the defeated boss will not count towards your trophy because YOU were not the one to kill it.

Do the rince and repeat, it works so far.