Its Triggering Unit, so its the unit who casts the spell....not involving the others..
And add the Ability "War Stomp (Neutral Hostile)" from the Unit tab....
And if you want the thing to blow "ALL of it UP"....its as easy as the Trigger I just made....
Again, Make An Ability Based on War Stomp, name it to "blowitup", and set..
Area of Effect to 0...
Damage to 0...
Distance to 0
Mana Cost to 0
Its kinda different now so...
Event
A Unit Owned by Player 1 Begins Casting An Ability
Condition
Ability Being Cast equal to blowitup
Action
Kill all Unit of Type Farm for Triggering Player
"..Oh no what have I done....please forgive me, I only necroposted because I wanted to help the poor guy from not getting his question answered...."