I've found it reasonably easy to avoid as a mage with fade step or a rogue with evade by just sprinting and dodging whenever he disappears but that is usually at the expense of being able to assist properly with the adds. Only times I've been in party that beats it on anything higher than routine is where the rest of the team were competent at cleaning up the adds fast so we could leave the DC to last. What is especially annoying is that it can one-shot tanks too, making the warriors almost worthless vs demons. I was enjoying playing legionnaire and tanking everything until I faced DC on threatening and seeing it one-shot me through guard in a way templar commander never could.
Had the best success against it yesterday playing as Arcane warrior and sprinting around when it disappeared with some fade-stepping if I thought it was about to hit me/I got hit and wanted to avoid the follow up scream. I would also occasionally drop Pull of the Abyss to help the aoe guys kill all the adds really fast. However, I dislike the idea that I have to constantly stick to a certain character type if I want to be sure of finishing missions.
The thing is that templar and mage bosses are manageable for a decent pug so I don't see why DC isn't on a similar strength as them. Sure the attack is telegraphed but with all the stuff going on, it's easy to miss and seems out of place in damage with every other boss stages. Right now it's near impossible to get a pug Perilous match going as I'm guessing no one wants to take the risk of getting demons on that difficulty. I'm sure a private group can stomp DC easily but tbh this game's multiplayer isn't active enough imo to get good private groups going.
Two things for me that need to be changed on DC:
Ground jump ability damage needs to be toned down a bit, since there aren't any other attacks that can one-shot so rapidly regardless of where you are (maybe like 25% less damage)
Needs to be physical stunned/interruptible in some way when doing slow attack or have some way of breaking free of the lengthy panicked status (at least on routine)
Also, venatori mage commander attacks need a buff, her attacks are pretty laughable and she just gets treated as one of the mob. Maybe have her attacks do a lot more elemental/dispel effects.