I have EXACTLY the same problem. I created a custom faction with no strengths, only a weakness, and that helped, but even with that race I generally crush expert.  

I have created a custom faction I think is "overpowered" without relying on an exploit. Got steamrolled against all ridiculous AIs, but that was just one game. 

I have tried playing with ONE opponent set to ridiculous, the rest expert but somehow one of the expert AIs beat him up, and I easily finished the ridiculous off.  

If you find a fix, let me know. 

You copy CoreDifficultyLevels.xml into the mods folder, then just mess with the ratio values for HP and economy, right?

I am pleased that I am not the only one to notice that the difficulty ramps significantly   I wonder if AIFOWCheat being turned on (I presume this is fog of war cheat, so that the AI can see the whole map at all times) for ridiculous is more important than the incremental economic improvements.  Sadly, I don't have the time to see how altering it affects the AI's playstyle.

They also get a lot more trait slots.

Try insane, thats were the real fun is.

I thought that even expert is not fun, because the AI gets high bonuses, but ridiculous (world and AI) is a walk in the park with Altar and a custom sovereign:

- Choose the General profession, Air 2, Life 2, Brilliant and one weakness

- Cast Tutelage on your sovereign and select the Path of the Mage

- Select the summoner specialization

- Design one Henchman with Potential, Water Adept and Bard

- Select the Path of the Mage and the summoner specialization

- Now you have an army of up to two Air Elementals, two Earth Elementals, two Fire Elementals and one Lightbringer