| Blackyce |
I think it would be wasteful to tackle the army in battle. In my opinion the smart way to achieve this would necesarily be with kidnaps/assasinations of important individuals.
Being a powerful wizard the smart way to achieve victory would be to make the armies disband. An easy way to do it is to destroy the command structures of those armies. What the wizard does is scry the leaders of those armies, teleport there and murder/kidnap the leader without raising alarm. Then he polymorphs into the leader and summons all his officers for a meeting. Then he blasts them all with a couple of spells. Rinse and repeat for every army and there will be no opposing armies in a couple of days.
After that he teleport to the capital and kills enough members of the goverment/rulers until they surrender the power to him.
I don't see how they can defend from this if the attackers have overwelming magic power like this. Probably a lower lvl wizard could pull this off as well if the persons to kill are not very powerful themselves.