You can probably use unholy aura (which increases regeneration rate). Just shift-click the regeneration and field and try putting a negative value. And then change the targets allowed to enemy units.
It is just a random guess, but I assume it would work. AFAIK, Heartstopper Aura is considered negative regen, not damage, so it was probably made in a similar way.
Make sure that negative values in object editor is allowed. Go to File -> Preferences -> General, and tick "Allow Negative Real Values".