Well for anyone who is curious, I thought of a way possible, but the only downside is that for example if I make the Catapult enable this, then it couldn't hurt itself, but it couldn't hurt other Catapults as well (friend or foe). If you wanted to do this, make it so the catapult has a newly defined armor and attack. The attack would penetrate other armors as normal, except make it so that the catapult's special armor receives 0% attack, or atleast lower it down. This way the catapult won't kill itself. I suppose its useful for some specical faction unit if you were making a mod.