Well, if you want to win, Wish for a "Doom Horror" or "Vastness". Two very powerful monsters. After that you will be very, VERY nervous if someone casts Wish, especially if that someone is human... AI does Wish for things that bad.
I think that your enemy suddenly lost almost all his mages, and had only a small bunch of undead mages left. As he had almost nothing to use his gems on and was too short on cash to be able to buy mages that could be useful he used alchemy to get more money and Death gems. He then either summoned more undead mages (maybe Revenants) and with them casted Reanimation, or Demiliches (Lichcraft) and more powerful summons.
Or maybe he just had so many provinces that even with 10% income he massed huge armies on the provinces near your border.
For masses of undead you could have survived by casting Thaumaturgy spells Control Dead (
)and Wither Bones (high armor-negating damage, no resistances or saving throw, only vs undead). Also, Earth spell Blade Wind does wonders against all low-protection units your enemy might have.
I haven't yet tried to play a game that big to the end... It seems that it will be interesting.