When I ran this, Xanesha barely escaped the fight on top of the Shadow Clock. That was a fun and difficult fight for the party. But, the next time they ran into her, the party was 1 level higher, with more gear, and they completely wiped the walls with her at Fort Rannick; it was quite a letdown.
Like others mentioned, if there is that much time between the assault on the Shadow Clock and the takedown of the Cult/Ironbar, she should have time to really get to know the party's tactics but ultimately, she should feel she can take 'em with the right preparation. I, personally, dislike removing prepared battles, especially with the BBEG of the book, even if, logically, the party will stomp over them. It is just like PC hiring NPCs to help them with battles - the PCs are the heroes, they're the one supposed to do the hero stuff.