Mythic Hans and Frans Solo
Unfortunately it seems a bit buggy (you can check mage and DH threads for similar complaints) so you can see I died after the achieve popped and game recognised i killed the encounter but you can see in the screenshot 1 of them still has health bar above his head (maybe cause i was tanking them against the wall) and their corpses actually despawned right after the kill.
But all things considered i got the achieve and my save says they are defeated so I will count it! Quite a tough fight, where I had to learn how to do the tough phase roughly halfway through without potting or lusting to save those to burn at the very end. Barely beat enrage timer (they actually were enraged for a few seconds). 90% stagger is quite powerful though. 175k hps or so.