Wizard’s Sign and Armageddon.

Please don't mislead people; the mage tag doesn't work on area-of-effect spells, i.e., Armageddon, Unholy and Holy Word. It does work on Meteor Shower.
P.S. Armageddon can deal damage in the right hands: https://www.youtube.com/watch?v=WeE0yhwckKA

I might be wrong; I apologize for the misinformation. Armageddon and Meteor Shower deal too little damage, so I rarely use them.

P.S. Where can I find people or groups to play online with?
I used to play Heroes 3 and 4 online, but I haven't had the chance to play Heroes 5 online yet.
P.P.S. I watched the video; theoretically, it's well-executed, but practically, it's not feasible. Besides, in the presented situation, Armageddon would lose to a Berserker.