If you placed the units fairly close together, they'll be in 200 range of you when your other mob is. You will need to trigger this so that the other units are ordered to stop regularly.
Not to be discouraging, but you'll need to know pretty much everything in WE to make an ORPG. It's like making a mmo when all you've made before is a Hello World program.
Command aura with a negative value should work, though you'd have to figure out how to only apply it to that unit. I'm not able to access the world editor atm, but you might also look into attempting to use a dummy ability and making it apply an aura with the only target as self (may require...
There is a way to do this through using a separate unit, similar to how dummy spells work. Copy your first unit, and edit the attack range to be different on the copy. Then you'll need to trigger the change (I'll post that trigger after this game of Angel Arena. :D)
EDIT: Here's the trigger I...
AI Scripts are used in campaigns to control your opponent(s). You shouldn't need them to play simple 2v1 or whatever comp-stomp games. The built-in scripts for computer opponents do that normally.
couldn't you simply create a trigger to deal the strength's amount of extra damage to the unit when the non-hero one damages it? (you would likely need GDD or something for this)