the ai may be scripted but that does not mean you cannot randomise the units that trigger ai.
The other thing you could do is randomise the missions on the map. So you program the map for 10 different AI tactics, randomise the tactics and then randomise the vehicles within those maps. Valla many different possible battles.
Also the AI will make decisions in battle to make things even more different.... the AI I believe is still supposed to react to you and fight you, counterattack etc on its own once its been triggered.
For instance the AI may start a battle with a small group scouting, they spot your forces and then the AI takes over and does what it likes. I'm sure it may even call its buddies in the rear to come up and take part