Doug Piranha
Warlord
The important promotions most of the time are shock, cover and formation. That is what allows a unit to be a counter to another. The AI seems to tend to get combat three before these promotions but, for example, a combat one and shock warrior is the favorite against a combat two warrior.
Most of Horatius's pointers are good general rules for the AI, but I disagree with this one. Once Horseback Riding has been discovered, every single unit should automatically receive the Mobility promotion ASAP. Also, in taking the anti-unit promotions, units should be tagged to specialize in only one area. In general, a unit should never take more than one of the shock, cover, formation, or slayer promotions. Otherwise, it's better just to take Combat promotions. There are some exceptions: for example, a unit could take combat 1, shock, and undead slaying for a +100% bonus against skeletons, spectres, pyre zombies, diseased corpses, etc.