Coop against AI?

[Edi]

The Better Independents mod makes the AI significantly tougher because it eliminates the possibility of hiring chaff troops by raising gold cost to 9000 for them. Use that and set AIs on Impossible and choose a fairly large map with a good number of AIs. Such as Glory of the Gods Multiplayer with around 2 human players and 9 or 10 AIs. Set indies to 7, research to hard, supplies 150 and see how far you can get. Powerful magic isn't too easily available and you don't get SCs all that easily either (unless playing one of the nations with recruitable SCs) and if you stick to using mainly standard armies backed up by combat magic, you'll be in for something quite different than a regular "ho-hum, AI go squishy-squashy" game.

With that setup, I lose more games against the AI than I win, especially if I start in the middle of three or more AIs.

You can find the Better Independents mod from the link in my sig.