You don't need any cd to survive the smoldering devastations. Just put your teleport up there, jump down when she's about to cast devastation (1s left) and teleport back up instantly, no damage taken!
You need to get her to 10% before the 3rd devastation, as she will go down into P2 at 10% regardless of timers. Then you just burn her under Dark Bargain, as all the spiders down there will quickly kill you.
The trick is to use Alyrazor buff to quickly kill Rageface and Riplimp. That requires pulling Alys in a certain time window, so that you get your 25 stacks when Shannox is coming nearby, and when Alys is about to cast Molting so that she despawns (you don't want to fight Shannox while Alys fight is still engaged).
Here's a video how how I did it , but you won't see clearly the pull timing. You need to clock how long it takes you to get to 25 stacks, and how long is Shannox full patrol path around, and pull Alys according to that.
I hope you didn't kill Alysrazor this timer