I agree pizzaguy

the AI is least hard to play against when it throws half-stacks of badly led and badly equipped troops at you on a seemingly random whim

this may be because it re-assesses the situation every turn and makes no long-term plans

I would like to see it (eg) decide to invade my eastern territories, send spies to check how well they are defended, make an appraisal as to what it needs in order to take them, train an army to do the job, then send that army led by its best available general to take those territories or die trying

that would constitute a challenge