So, you have 40 Post-Imp Mongol Seige Onagers and your job is to destroy a wonder.
No sweat? Ha!
Everytime it goes down, it pops back up!!
But, destroy the magic tower to the west and the spell is broken. The wonder goes down, it stays down, you win!
I think it's a nifty trick that hopefully hasn't been posted before. Try the scenario first and then veiw it in edit mode to see what triggers it uses.