For sub rogue i ended up just burning 2 phases of adds and then killing boss after the third set spawned but before they reached him. i was using shoulders+cloak, if you vanish with 30 stacks you can just about 1 shot a wave and then you only have 1 wave to nightblade.
for the Havoc challenge the guide i read recommended going chaos blades / felblade after the xylem part, but i tried that once and didn't quite have the burst. going demonic build you can just save an eyebeam and then a deathsweep and the adds are pretty much dead, i fucked up on the third wave and wasted a fel eruption early and had 2 adds get through and heal the boss, the entire platform was covered but i was able to pop meta and just stand in the bad and kill 20% of his hp.
the tank challenge can be shitty with the knockbacks since its not always exactly where it puts the graphic (for the infernals) and the eyeballs are oddly specific at the angle you have to face. for the DK i ended up downloading a WA that had a big airhorn whenever a knockback was about to occur and a different noise when the eyeballs were casting. I think Prot pally is still the harder of the tank challenges atm. I would be interested in trying BrM again now that i overgear it, i dont remember the nether adds actually casting their AoE on the BrM challenge either, so either they dont have that ability or i killed them too fast.