One thing I'm noticing with big battles is that, if odds are fairly even, the AI will hang back near it's own reinforcement zone. This is smart, but it makes for bad battles, because the flow of the battle will be sort of
1) Player Army advances, beats the socks out of the AI
2) AI reinforcements arrive, overwhelm the player army
3) Victorious AI troops advance, and run into
4) Player Army reinforcements, which overwhelm the AI army
5) Victorious Player Army advances, and run into
6) AI reinforcements arrive, overwhelm the player army
and so on and so forth...
While it is not "smart", like in maximizing their chances to win, I think it would make for better, less gamey battles if the AI generally advanced to the middle of the field, even when feeling defensive. Size good ground and stay put, sure, but not at the map edge.