Need help with playing against the AI!

Don't listen to Ment, he's clearly not very knowledgeable about these things. Choose Naadir, you won't regret it. Be sure to learn mass plague and mass dispel, evasion, and battle to the last. You can also add enthrall with a phoenix – as a bonus. No skeleton archers, only warriors. Try to shield as many as possible with them and don't go to the front lines. Focus your liches on the archers; if they attack, cast plague. Once the archers are dead, block the druids. Slow down the trees that remain at the end – they're practically useless meat. Use ghosts on unicorns, and use drakes and messengers to eliminate enemy drakes. And most importantly, don't be afraid of these elves; the only danger is the spell penetration they can get from a mentor. Good luck!

Yes, and don't rely too much on vampires – the elf will almost certainly mark them as enemies, but still, try to blind as many as possible before they die.