[PvK]

You can set them all to impossible, up the supplies, and choose settings that don't favor your own play style. Crowding with many AI's gets you into early wars which limits use of as many fancy high-powered magics which the AI isn't good at. So does adjusting other settings that favor combat over magic: high gold and resources, slow research, low magic sites, and mods such as my "Price of Power" mod or Illwinter's restricted magic mods.

Limiting your own use of tactics the AI doesn't use helps a lot, too. I like to choose a thematic play style for my nation and stick to it. Or if you are interested in "fair play," don't let yourself use tactics you know the AI doesn't use effectively.