That's probably the major flaw in the AI. It's quite happy to sit there and let you pelt them with arrows. Rebels are the worst for this.
Although one of my favourite moments was when halfway through pre-battle manouvers an enemy general decided to charge my Triarii. With 20 cavalry. While the rest of his army looked on and did nothing.
I have no idea what he was thinking really.
I think one of the best things about the game is the historic battles. Trebia is probably my favourite. Charging elephants vs roman legions
I'm seriously tempted by the Alexander the Great add on
I'll have to check the reviews.