you can give every hero a critical strike ability that is 100% but does 100% damage and put it into a disabled spell book
I have a tutorial of Damage System.The map arent in jass...so you can learn it easyly.Another great point of the map is:The hits damage appear above enemy model and you can read the trigger to learn and do what you are looking for(Just heroes for example) so you can copy the trigger and make a condition of unity-type = hero or maybe a variable.
reply if you want to explore that tutorial!
Create floating text that reads <triggered - damage taken> over attacked unit