I did the bear challenge after 6 tries, I didn’t use any of the gear that was suggested, I also didn’t use that utterly awful spec, just use M+ elune spec and spam moonfires for an eternity, I went in with half hero, half mythic, 710ish ilvl with M+ tank trinkets a flask and weapon oil, it really wasn’t that hard, just don’t panic and focus on controlling the fight.
On my winning try, there was only one infernal active, which made it easier.
Moonfire is your strongest tool here, you only need to pop one on each add and they will die on their own, keep the main spell interrupted on the big guy, don’t get your debuff stack too high as it lowers your max healthpool, spam moonfire on him when you can’t sit in his bad to melee.
If the adds are starting to cast, you can use one of the orbs Velen drops or your aoe interrupt. As long as you moonfire adds when they spawn in, they will gather themselves close enough to you for the ints to catch em all.
When the boss comes down, just walk backwards the whole fight, make sure to interrupt him and use your defensives properly so you don’t get one shot.