Use the following action instead of the
Unit - Set life of (Attacked unit) to ((Life of (Attacked unit)) / 2.00)
action.
Unit - Set life of (Triggering Unit) to ((Convert real to integer: Damage Taken) + (Life of (Triggering unit) / 2)).
There are 2 notes however: the "Damage Taken" real value can only be used when responding to a "Unit takes damage" event. Strangely enough, I can't find this event right now... Tell me if you did. Also note that if you're using the new event, you'll have to change "attacked unit" into "triggering unit". Besides, with the new event, it's possible that this trigger will also run when the attacking hero is casting a damage spell..
Second note: I don't exactly know what will happen if the damage done by the hero is larger than the actual HP of the attacked unit. However, I believe the attacked unit would still die.
I'm going to sleep now so I'm too tired to open world edit