This thread should definitely be stickied, because of Saber Cherry's post. With spoiler warnings.
Isn't that a bit drastic?
Do:
Hire mercenary mages.
Hire independent mages and
especially sages.
Give gems to your mages for important battles.
Script your mages.
Get a level-4 priest to cast fanaticism constantly.
Use fast, flying and cheap units to patrol.
Build lots and lots of scouts and spies.
Protect your mages with a screening force.
Don't:
Engage in a guerilla war against the AI.
Take Golem cult.
Use top-tier mages for research and nothing else.
Use sages in combat.
Try to assassinate a death mage with a bog-standard assassin.
Enter the arena with anything valuable.
Give your mages heavy armour.