My knight is now around level 170 but I was about 150 when I killed Kalyassa with him a couple days ago. It is by a good margin the hardest miniboss to kill in my opinion and since it was my first time attending the First Dragon event I was completely clueless.
With that said I had no strategies, so all I did was attacking doing spell rotations and standing diagonally from it. I also made sure to hit the summons, including front sweep.
It's definitely doable by but combos can hurt a lot, so be careful.
Good luck!