Find an ability that gives a percent regen bonus, then make it so it gives a -100% bonus to hp regen (thus making the regen 0). For setting the unit's hp to a percent just use the action that sets a unit's hp to a percent (yes, it exists lol)
However, if you can't find it or it doesn't exist (been awhile since I used GUI ), just pick every unit on playable map area and set it's hp to Max hp of (picked unit) * (your percent converted to a decimal, ie 25% = 0.25)
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