[General Tacticus]

Something that has been mentioned before : impossible AIs do relatively poorly on small maps, because they are trying to build for the long term (researching, site searching, ...) and will be rushed by a player who knows the map is too small for that time frame. he AIs are also slowed more than the player by high indy settings.

Playing on small map, you could try to set the AIs as allied, or give them a mix of difficulties (hoping for one or two to gobble the others and offer a challenge), and put indies to 5 or 6, no more.