|Sand Von Glokta|
This encounter turned out to be one of the best we've had so far. I had to tailor things a little (although most of it's hinted at in the text):
It mentions in the introduction that the taverns and flophouses throughout the Shadow district have bets running on the inevitable collapse of the Shadow Clock. My PCs passed the Friendly Merchant tavern on the way to the Clock, where they saw the chalkboard of odds on where the tower would fall, and heard some of the stories about the tower's "haunts".
When the bell crashed down the staircase, I stressed how much the whole tower shook and creaked, as though it were liable to tumble at any time. Lastly, I changed the layout of the roof so that the angel was perched atop a "witch's hat" rooftop, with Xanesha's lair in the attic beneath (which is kind of how it looked in the picture). I told them that almost half of the roof had completely fallen away and was open to the elements, and the rest was dotted with missing tiles. The statue at the top was supported by a single, ancient pillar of wood. Just to make sure they really got the picture, I allowed an Architecture and Engineering roll to determine that the statue could potentially collapse the floor of the attic if it fell onto it.
To ensure Xanesha's demise they'd have to lure her down inside the tower or directly beneath the statue before toppling. I figured she'd naturally avoid slithering beneath the statue, and wouldn't enter the tower beneath so long as there were enemies to kill in her lair. If they could lure her inside, then collapse the statue, it would crash through the weak floor, through the gears room, through the bells, and bring the whole structure down on her head.
I guessed Xanesha would chase the PCs inside if they fled down through the tower. If one of them could hide in the gears room, he could double back upstairs after she's passed by and topple the statue. The others could barricade Xanesha inside by spiking the main door, then wait for their ally to fly or featherfall down the tower after it falls (or die in the collapse - one death is better than a TPK!).
As it happened, my group tried to take Xanesha on head-to-head - and suffered her wrath. Eventually their fighter was slain, so the cleric grabbed the rogue and dived from the roof, using an Updraft spell to land safely. The wizard fled inside the tower and hid (assisted by the cloak of elvenkind worn by the Scarecrow!), then cast an illusion of herself running down the stairs. With no reason to disbelieve, Xanesha slithered down after the illusion, giving the wizard a chance to scramble back onto the roof and topple the statue. As the tower began to collapse, the rogue/sorceror cast a benign transposition on the wizard and her hawk familiar, bringing her back down to ground level. Very cool!
Anyway, hope some of this may be of use to someone :) I'm with the original poster here: going toe-to-toe with Xanesha is lethal at this level: even with buffs and 'aid another' your melee PCs are unlikely to get many hits in. Even if they do land some strikes (and beat her mirrors!), they've got to wear down roughly 130hp just to make her flee - and every other round one of them is likely to drop.
Her high spell resistance puts spellcasters on a similar footing. I agree that true strike could help, as could dispel magic (if you're lucky!), but you're screwed if you don't have those spells on your list (or you've already used them up). I understand that forcing a retreat against almost impossible odds is fun once in a while, but I'd argue that it's poor placement at the finale of an adventure. A fight on the very top of a crumbling tower, against an opponent who's responsible for nearly everything that's happened to date - that should be an epic, hard-fought win, not a sour retreat!