Figuring out the timing for when he is going to mirror images was the single most time consuming part of getting this guy down for me. It was the difference between 10 attempts and the 43 it actually took me.
However, there is actually some predictability to his behavior and when he is going to switch phases, and if you pay close attention to it and figure out the timing, p1 becomes a breeze.
The key takeaway is this: boss mirror images every time he phase changes.
If you tunnel boss when he's phase changing, feels like there is about a 90% chance that he'll port far enough away that a VR+FR+FR wont get you to him ontime.
The trick to handling him reliably every time, is this: when you think you're getting near a phase change, stop hitting him. Theres no dps check. Run to the middle. You should have fel eruption and consume magic available to interrupt his casts, and if he gets 1-2 off it shouldn't be a big deal. When he phase changes, he always starts the cast of the new phase before he ports. So say you're in frost phase, if you see "arcane barrage" start casting, you know that you have about 1/2 second before he ports. If you're sitting at a ranged distance right in the middle of the room, you can throw an imprison on him right as he starts casting it, and then when he ports, its very easy to see where, and 2 fel rushes are enough to get you to him 100% of the time from the middle of the room. You dont even need to get tricky with interrupts this way.
Once you get this timing down, phase one will go from being "I got lucky and saw phase 2 once" to "I could do phase one for 3 hours pretty easily." Its very rinse and repeat, and nothing should ever surprise you. I dont even meta in phase one because it just isn't necessary and it throws off my timings for frost phase.
Speaking of frost phase, here's a couple tricks you can do: if you do it right, you'll never use any interrupt other than consume magic, and you'll only have to use it once per port.
Take unleashed power for the faster, no cost chaos nova.
He imprisons you in ice. AoE stun, double jump over to him, interrupt, and drag him toward middle. Eyebeam the pile of ice for an instant eyebeam reset and demonic activation. DPS boss for 8 seconds. When demonic falls off, you should have another eyebeam up. Eyebeam boss. dps him for another 8 seconds. He then ports, and you stun and repeat. Interrupts and paying attention to casts becomes negligable, leggos dont matter because you're sitting on like 95% uptime on demonic, and you can save all your interrupts for arcane phase.
Phase two, make sure you take master of the glaive and bloodlet, bracers or not having that slow is vital and three target cleave bloodlet beats out first blood. The dps check in p2 isn't that intense.
Edit: I would recommend making a WA that tracks if youre standing in the crushing darkness. Its like a 10 minute boss fight and having an attempt go down the drain because you tunneled the boss slightly too hard in p2 is a major downer. Mine made an air horn noise at me if I was standing in it.
Edit #2: Time to start the tank challenge! RIP getting a chance to make attempts at it before nerf