Need help with playing against the AI!

There's a lot to say about this. Perhaps the ideal option is Necropolis – capture as many towns as possible in the shortest amount of time, build a converter, and multiply a huge army, buying troops from different towns and turning them into undead. It's also worth exploring summoning spells: Phoenix will be very helpful, and Celestial Shield will also be good. Don't bother with Darkness; an Elf can create Battle Unicorns, and with a bit of luck, you can also get Reflection.