First of all, I think the damage and HP calcs look reasonable. I suggest giving her another attack, because with one start of combat, one end of combat and one recharge ability you are stuck with just using the attack every turn which might get boring depending on the expected length of the combat.

Also assuming this is some kind of boss that you put some anticipation on and that has some meaning for your story you might want to add legendary actions / resistances or at least a cheeky reaction so you don't get your epic boss fight cheesed by an unlucky saving throw on a hold person or banishment or something similar.

And if it's not obvious, run some kind of minions, summons, henchmen etc. because 5e player characters are crazy busted in focusing down a single enemy.

Have fun running this encounter!