I would restat Reymus as a Level 22 Solo Soldier, throw in some excellent counterattack powers and so forth. Then I'd stage the scene so the primary goal is to not fight him.
Three skill challenges, I'm thinking...
1. Get the Grey Guards in the tower to safety / Mobilize them against Reymus. Failure leaves them vulnerable and unprepared, so they drop like flies when the actual battle begins. Success gets certain key NPCs out of the way (the weak support ones who will perhaps pick up the pieces later) and positions some of them to fight (man the siege engines, form up a pike square, create a roving search party to find egg sacs, etc). Avoid alerting Reymus to what you're doing or he will interfere; failures mean incapacitated NPCs who can't escape or help when things get really bad.
2. Find and destroy some of the egg sacs. Bonuses/auto-successes come from "organize" track of earlier challenge. Failures mean a certain number of sacs go unfound/undestroyed. Even total success doesn't destroy all of them, just a majority; with a zone map and some planning, PCs might be able to pick their ground for the final phase by forcing Reymus to defend one particular area where the eggs remain undestroyed.
3. Survive and escape Reymus. In a setting with plenty of opportunity (holes in roof, hurricane winds, sacks of supplies, mirrors everywhere, siege engines, etc) each PC makes a skill check every round to avoid getting seriously hurt. Some skills stop working after someone succeeds two or three times- Stealth won't work after the first two successes, because he'll be ready for it, and he smashes the walls Athletics was taking advantage of later. Any PC who rolls a failure takes 3d12+20 damage or so, described in various ways (Reymus casually backslashes you with his sword, kicks a crumbling crenelation onto you, gets a hand on your neck and you barely break free alive) appropriate to the skill used. While this is going on, PCs can use their actions to smash more egg sacs or mirrors or otherwise mess with the plan- but if they're doing that, they're also getting damaged. Reymus seems more like a force of nature than something they can fight- though they are always free to attack him. If they do manage to daze him for a round, etc, only one PC has to make a check for that round to avoid the damage and the others can all make checks to find/destroy more eggs.
I'd be comfortable adding a few specific details and rough maps (so you know which PCs are in which part of the tower, where Reymus is, etc) to the idea and then winging the rest of it on the fly, but that's a question of style.