You could manually add hardened skin whith many levels to units, for exampls 45 means level 45 hardened skin. To hide icon make in in a spellbook ( if ya dan't know hoe look it up in tutorial section). There isn'T a maximum damage anyway, if the editor resets you numbers, just ctrl click on the ability.
In jass you are able to find amount of armor, in GUI you have to listen al abilitys buffs and base armor up to get armor(rly anoying)