I'm rank 5 at the moment and Millie Wat was an absolute nightmare. I spent just over an hour before I finally killed her and then as she dropped I died to the enrage timer meteors. Not fun.
Fjoll doesn't seem like he's that hard but I'm so absolutely burnt out I have no desire to put in the effort to killing him. Its amazing how steep the difficulty ramps up for this in what feels like a short time.
I've been told by some people that to advance beyond rank 6 there is actually a minimum DPS needed before the enrage timer goes off and to achieve that you need to have decent PVE gear, which makes me a little sad but I'll get as far as I can and hopefully once my gear is upgraded I can do a bit better.