Help with passive custom abilities

Hey, i'm making a map and are making some custom abilities. Basically I want to make a passive ability that acts like defend (reduces piercing damage) but is always active (hence a passive ability). I was wondering firstly is this possible?? And if so, how would i go about doing it (ie can i do it in the editor, and do i need to do slk stuff, or do i have to use triggers?) I'd prefer to avoid triggers if i can, i know how to do it that way but since this ability will be on lots of units i don't want heaps of triggers running to check and modify damage every time a unit with this abilility is attacked).
Any help would be much appreciated.
(Oh and i know that you can do a ranged damage reduction using hardened skin, but i'd prefer to use the defend priercing % damage reduction if i could)

Use a modified Elune's Grace.

Use either a modified Elune's Grace, or Hardened Skin.

Use elunes grace, cause hardened skin has some interesting side effects in regard to certain attack types.

Thanks for the help everyone. Just so i know, there is no way to edit abilities to make them passive?

The abilities are what they are. The only things you can change are the graphics and some numbers/flags. Some spells allow for more costumization (for example, there's a lot that can be done with silence), some less (on robo goblin, you can't even change the bonus armor and strenght), but essentially, the basic layouts of the spells are still the same.

The Bonus Armor and strength are part of the 'Engineer in Robo Goblin Form' unit itself. ^_^

Ummm... Where in the object editor do you see that, exactly?

You see, that "robogoblin" ability is like avatar, give strength and armor

By comparing the normal Goblin Engineer to it's "Special" counterpart (Robo form).

K thanks heaps everyone.

When I compare them in my object editor, they are indentical (armor and strenght) except for that one of them has the mechanical classification.

yeah, Robo Goblin's str and armor additions are set into the .slk, you cant edit them.