First..in an average campaign, at this point in a pnp game, your group is amonst the most powerful forces in the humanoid community. There are not many level 16+ people running about. That creature you are going against, ist the most powerful fey you can usually encounter..they are almost godlike.
And yes, I surely hope they start the fight with greater dispel magic, because they are highly intelligent. Next is baleful polymorph to turn all your insolent casters and rogues into dogs.
If that greater dispel got your protection from evil (which I do hope you have despite them not being evil...) then welcome to dominate monster once (maybe on your strongest looking animal companion)/charm monster at will with a DC 22 will save. Hope your fighter is up to it and your mage who is not yet a dog.
You call out the "randomly paralyzing" thing. That is a dc 31 paralyzing/terrifying/confusing gaze. He can do that as a free action. FREE as in beer, each round as many times as he likes (unless the game imposes a limit). Yes, that is the paper version of that monster. Nothing random.
And if he kills someone of your party, he can reincarnate them as a free action as a FEY. They are gone forever unless you kill them AGAIN, cast break enchantment and then raise them again.
Now the fun part...even if they do not cast spells, they have an all-bane +5 thundering glaive (reach..haha..i am not stating on that) which he can toss out 100 feet and have it return as a FREE action.
But why should he do that? His spells and abilities are so much more formidable than his weapon.
And if you give him any trouble (should not work @ level 15 but the AI is dumb), he would just summon a CR 18 encounter once a day and while you deal with that (which in fact is almost as powerful as the monarch himself), he sods off to return after recovering.
I could lay out how to beat the monarch in a pnp game with a well equipped level 15 group. If I could actually put him down for good @ that level....I have my doubts. Not without losses unless lady luck really smiles.