It's not bad, but it doesn't feel nearly threatening enough to me for a CR 13. It should have greater than human strength and agility (at least 22), and should have enough offense to one-round down a caster-type if it gets all its attacks off.
I would drop its hit points down to 100 or so, but give it regen 50 if not hit with silver (or some other discovered weakness).
But I'm also generally of the opinion that lycanthropes (other than wererats) should be more "puzzle monsters" than just stock brutes.