I agree, it's hard but not in a fun way, dying to invisible things doesn't feel good.
In case ppl don't know, because didn't try it, boss heals on kill by big % of its hp, so while being a fight that pretty much requires a party nobody can die. Hp pool is massive so even perfecty executed fight will take quite some time, and invisible shit kills you, thats most of the difficulty. Very hard, but not very fun.
You can reduce its speed by a ton (using certain gear and skill combinations in party), but it still doesn't solve the problem, he will cast stuff many times before you are able to kill it.