One question is: Do you want your PCs to defeat the bad guy now? If they do, what next? If your campaign is setup for this guy to be around for a few more levels then finding away to let him escape without a TPK should be a key.
Also, unless the PCs know about those eggs, this sounds like a lose-lose situation. Either the PC face this guy and die (loosing) or they defeat him, but the eggs hatch (PCs still loose).
Sadly, he's sort of behind it. He's ushering in psychic monstrosities that live in mirror reflections and which are possessing people. The tower roof is full of egg sacks (albeit ones that can only be seen through a mirror), and Reymus is trying to buy a little time for them to hatch. It's not critical to his plan, though, and he can write them off if worse comes to worse. Still, he firmly believes that he always wins. Always. It's a flaw.
If you want the bad guy to escape without wiping the PC, you given yourself an out here already. First, you need to find away to make the PCs, and their 30 allies seem dangerous to the bad guy. Perhaps one of their new allies has a ritual that will let them harness some of the power of the hurricane to their benefit.
SO the PCs and allies confront him on the roof. Some monologing ensues where each side says how powerful they are blah blah blah. The bad guy tosses a mirror to a PC allowing them to see thre eggs. then gives them a choice: "Fight me now, and these eggs will hatch before you can defeat me. And what's in those eggs will wipe you and this city out. Or let me go and deal with the eggs before they hatch."
Yes, the eggs still do have 2 hours, but nobody has to know the bad guy is lying.
If they fight him, you're back to where you started, but with a way to make the fight not a TPK, if they let him go, your bad guy lives for another day, and the PC have another reason to hate him.
And if they let him go, then figure out the eggs weren't just about to hatch, so they did have time to kill him and take care of the eggs, they will hate him even more (and maybe you too).