So it seems like we may be fairly good at soloing dungeon bosses in Legion, thanks to Obstinate Determination and either really quick strafing reflexes or the clever use of room geometry. I soloed Hymdall in Halls of Valor tonight. The fight took about 6 minutes and at no time was I in any danger of actually dying. I'll give some higher difficulties a shot tomorrow and Tuesday when my respective lockouts finish.
I've linked a video of my kill below, but be warned, I play on a fairly crappy laptop and the screen recording software also chunks frames some, so it's not the most pretty thing.
https://www.youtube.com/watch?v=LntfooCgTyQ