Either periodically pick all units in that region and give it some defensive ability or use a damage detection system to remove a part of the damage done to units who are in that region.
Create a damage redution ability, it would be best to add it to a spell book then disable the spellbook for all players, then make a trigger, unit enters region, give ability (spellbook), then a 2nd trigger, unit leaves region, remove ability (spellbook).
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory