Allowed Targets Problem

Im trying to make a specif unit only attack certain sypes of units. I thought to myself "No Problem", Ill just make the unit I want it to attak a Ward or Ancient, and let it only attack a ward or ancient. Well, I set the "Targets allowed" of the attacking unit to "Ward" and the "Classification" of the attacked unit to "ward". I order it to attack the unit and I get "Unable to Attack Units". So I fool around with them some more, with little luck. What am I doing wrong?

Can you post the WE data of the target unit? If so, maybe I can help. Also, the whole "Targets Allowed" thing never seems to work right, so you may need a trigger work-around.

Even better, is there away to do this using a auto cast ability? That would even be more kick *** for I could hav eit hav ea "cooldown" period when It runs out of mana- with an average recharge. Im making an anti missile unit. It will only beable to attack missiles.