I don't know if you can do it without knowing the armour value of the units. You can manage this by storing extra unit data inside a hashtable (damage, armour, health regen, etc). These are extra stats on units that you can't just get a hold of with the triggers for whatever reason (for instance, you can get a unit's max health or a hero's strength, etc).
With a stat modification system, you can subtract or add as much armour as you want (not infinite, but probably more than the game can handle without breaking if you really want to push it that far lol). If you know a unit's normal amount of armour, you can make it so it cannot subtract more than that amount.
I am not sure what you mean in the second post.