The fight goes like how you described you need to kill that crystal as soon as possible. While also keeping an eye on Aramril as they channel an attack which you need to interrupt.

Which sets you are running and skills also? Also any visions you have accumulated throughout your run do not work there.

I keep my ultimate specifically for the crystal, but soon after it's activated, Aramril will do her channeling ability, so keep an eye to interrupt her asap. After the first crystal is gone, you can take your time and replenish ulti for the second one. If the hunger add is up I'd also try to kill that one next to the boss because the CC'ing it does is annoying. The boss usually transports to the edges and I think it's easier if there aren't any tentacle adds around her, so you can try to pool the tentacles in the middle, or on one specific side of the arena. But yeah, on the highest difficulty it's a tough fight, having a ranged interrupt can help, so does a gap closer (I used stampede to go back and forth between her and the crystal fast), also a shield for when the crystal hits become faster.

Sorry for the late reply, I just wanted to say thank you for everyone who gave advice. I was able to get past this yesterday and actually advanced on it while doing my first duo run last night :-)

Congrats on the win! Just one more tip in case anyone else comes here looking for help - she borrows some mechanics from the Runemaster Xiomara boss in Apocrypha, so that's potentially a place to get some practice with the mechs without having to repeat the archive hoping to roll Aramril again.