My own take on this:
The AI relies too much on battles of annihilation, bringing its whole force to bear in a frontal assault with the aim of routing my army. There are other strategies that it can try to adopt, such as attrition, raids and feints, but it is not doing these very well nor very often.
Lack of attrition strategies before a main assault means that my cities usually have a full pool of recruits. Even when I'm outnumbered, I have found it effective to send whatever nearby units I have out in a suicidal defence while bringing up reinforcements from farther cities. The AI will use all its forces to repel the sally, where it could have merely tied up my defenders and used the rest of its units to capture the city. The time bought is usually enough for my reinforcements to arrive and defeat the invaders, and my sacrificed defenders have also regenerated enough to join in the counter-attack.
The AI is also not conducting raids very well. It does not understand when to withdraw, so the raiding party ends up being wiped out by me. With the current recruitment system this can mean years of peace while the enemy regains its strength. As mentioned above, the AI will bring its whole force to bear on any counterattacks. This means that the raiders usually do no damage, as long as I can get someone there in time to attack any of their units.
The AI's current troop mix is also not ideal. I'm not sure whether this is historical and intended, though. Cavalry aside (since many groups have no access to cavalry), the AI's forces generally have too few hoplites and spearmen to form a battle line and effectively protect the peltasts. I have defeated armies that outnumber mine by using 2 units of phalangites/hoplites to tie down their spearmen, then sending cavalry to mop up all their peltasts before attacking the still-occupied enemy spearmen from behind.
On the plus side, the AI does a good job of exploiting badly formed battle lines, sending peltasts after unprotected catapults and sending cavalry after lone peltasts. The AI also does a decent job of flanking on open battlefields. Also, the AI's use of ranged units can potentially tilt the battle in its favour, provided it can properly protect its ranged units.
I'm not sure how much of this can be programmed, much less how much of it will make it into the release of Gold. But hopefully this gives the devs some implementable ideas.