Here is a video of my hunters clearing all Karazhan bosses except Nightbane. When the time comes for me to do a speed run, I have no doubt this team can easily manage the speed-run part

Anyway this was my first time doing this with my hunters. I had to read up before some fights since I have not done much Kara. Also the first boss was completely new to me.

I one-shotted most bosses, the ones which took some wiping was Medivh and of course Viz'aduum. Maiden is also quite technical and can probably be difficult if you don't handle the mechanics properly. But I had fought her a lot with some guildies as a single hunter, so for me it was an easy kill.

Medivh is just brutal. Luckily 5 hunters can interrupt quite a lot, so if everything is done good enough he should die just as your characters starts dropping aswell. I chose to interrupt all the frost and arcane spells, ignore the fire.

Viz'aduum was even harder. After a while, I found out that heroism can be used in the phase just before he teleports to the final position. By doing that and also using crows, he can be brought down quite low, allowing a shorter last phase which is by far the hardest. Heroism can probably be saved for last phase also, just make sure to bring him down as low as possible before last teleport.