[Boron]

if you want a difficult game you should take a small map with 150-200 provinces with all ai opponents and
use some modded nations as opponents especially cohens crusaders because they have supplyproducing leaders and very strong units so the ai doesn't suffer as heavy under starving effects as with almost any other nation expect ermor and carrion woods .
another "solution" for a harder game is using no sc-pretender and playing a heavy magic nation with bad national troops like tien chi s&a under very difficult research .

a good and easy solution would be if for the ai castlebuilding costs are halfed and supplies are multiplied with at least x4 . perhaps the templebuilding script can be used for castles too since the ai builds temples quite often .
it would be a small cheat but i think help the ai a lot .