Mimic the order of build style wouldn't be too hard, even if it just decides to proportion the same amount to barracks/ports/market as the player.

Battle style may be more difficult as then you would need probably some sort of neural network, then again maybe just having a wider range of tactics and having a history of which work best against the units you have deployed might be good enough.

Charge general across bridge won 1 / Lost 99
Charge Naginata across bridge won 3 / Lost 16
etc
The computer orders figures out which tactics worked the best in the past and randomly selects from the current best 3 or so. That way the player is up against a hard opponent but doesn't know what they are going to do.