[MENTION=12731]CapnZapp[/MENTION]: that's a solid game plan you have
Since much of Acererak's set up is dependent on the time stop, what spells have priority if, say, he only gets two rounds with it instead of the full five? Just curious.
With regards to the reverse gravity/prismatic wall (sphere)//lava combo (which I personally think is brilliant)..I'm going to put forward something that my players would likely point out: if the reverse gravity causes everything to "fall" straight up, and the party was on the platform and away from the lava at the start of the reverse gravity, wouldn't the lava also fall upward (like a waterfall) and harmlessly hit the ceiling and not the PCs? Sure, they would likely get some lava splashed on them, but would that still be full contact damage?
It's a nitpick, I know, but one I could see players mentioning.

With regards to the reverse gravity/prismatic wall (sphere)//lava combo (which I personally think is brilliant)..I'm going to put forward something that my players would likely point out: if the reverse gravity causes everything to "fall" straight up, and the party was on the platform and away from the lava at the start of the reverse gravity, wouldn't the lava also fall upward (like a waterfall) and harmlessly hit the ceiling and not the PCs? Sure, they would likely get some lava splashed on them, but would that still be full contact damage?
It's a nitpick, I know, but one I could see players mentioning.