Well the AI civs now defend themselves well.
Builder Civs are brought down a few notches because they can never build BUILDINGS because if they don't build constant military to defend themselves they get annihilated fast. Thus building 50+ military units per city is required for builder civs.
War Civs are brought down a few notches because all their potential targets are so well defended.
If there are 3 civs, each with 50+ units per city, then how could the war civs profitably attack any other civ?
Ultimately the only really good answer is to improve the diplomacy part of the code so that 2+ civs would work together to defeat an opposing civ. Thus Monte would have to work with say Isabella to divvy up say Rangar in a mutually beneficial arrangement.
This would be a MAJOR programming task. What if Isabella agrees to attack Rangar, but gets jumped on by 2 other civs after Monte has already committed to the attack and has lost tons of units to Rangar who now can focus completely on Monte because Isabella sent no units to help Monte? Or alternatively, would Isabella be programmed to sometimes just plain out backstab Monte, agreeing to war and then making friends with Rangar? This would require code segments that update each civ-civ relationship and have the civs act according to these relationships and also according to what their "overall civ goal" is. Which means that "overall civ goals" need to be created and implemented for each civ. AI personalities would have to be customized as well, along with backstab probabilities and negotiation "toughness" etc.
This is doable, but not easy. It seems to me that a good team could pull this off in 4-6 months.
Unless the desired answer is make it so that the civs just refuse to defend themselves so that single civs can easily take them over. Which pretty much is the original solution that Firaxis came up with.
Their solution seems simple-minded and trite. Except that, for example, the idea that "AI civs should not attack civs that are too far away" has had many game changing consequences that were not anticipated. The obstacles caused by the law of unintended consequences are sometimes too expensive to overcome.
Blake and his team have taken Civ to the next level. CONGRATS and WELL DONE
But they have accomplished so much that the law of dimisishing returns is kicking in, and further refinements will require ever increasing amounts of work with expanded risks of unintended consequences.