Holy fuck that took long. Here are some tips...
1) Repairs/Respec - You can HS out and repair/respec. You have to leave the scen group (won't let you teelport back in), but restarting the scenario ports you right to the end guy.
2) Make pitlord macros! - Enslave the pitlord as soon as it spawns. You can Sfury it and enslave it before it casts Charge and stuns you.
1 /use [@Pokeadott] Fel Flame Breath (self dispel and dmgs imps)
2 /use demonic siphon (the heal)
3 /use Charge (to stop cataclysm)
3) Demon specific how-to - YMMV, but this was by far the best way for me...
1st Imps: Stand on/near them and use breath to kill them. RoF for free embers. Careful about his RoF. You can Spam CB during this due to insane embers.
1st Fel Pup: As soon as he moves to channel the cast move your Pitlord to the FAR right edge of the room. Put an RoF on top of the portal and aggro the Fel Pups ASAP. Pull them left while Havok + CBing them down. They will outrange the Devour Magic!
1st (only) Doom Gaurd: Perma banish
2nd Imps: This is your kill window. Boss *should* be sub 20% by now (I'm ilvl 498, had flask and food, not the world's best Detro player for reference). Put down RoF/Pitlord breath DS/Pot and spam the fuck out of Shadowburn. You can chain cast it thanks to the embers from imps.
3rd Fel Pups: Rinse/Repeat ph1 if needed, ideally he's dead before or right when they spawn.
Hope that helps. Was a fun encounter for sure!