In order to make the final battle with the sorcerer king start before the players have sniped the mirrors; I've thought up a little change which might make things a little more tricky for the players. As long as the sorcerer resides in the mirrors he can guard (as the heroes). He loses this ability when the mirrors are broken.
To make things even more heavy on the players (I don't know if that would be necessary) you could have a special rule regarding the number of mirrors having line of sight to the targeted square (player). Having the ability to watch the players from all the different mirrors at the same time should give the sorcerer a combat advantage. For each mirror having line of sight to the player add +1 pierce to the attack, but you have to originate the attack from the mirror with the longest range to the targeted square. In effect, the king could choose a closer mirror but would then lose the +1 pierce from all other mirrors farther away. This would create a strategic element as the OL would have to consider high range and high dmg vs. lower range and less damage.