Well there's a couple things you could do, Didz. One would be to come up with more viable options and present the AI with a weighted random choice between them. Currently, though, the AI tries to evaluate the situation and deploy its troops in the one way it deems most tenable... so the implication is that if you give it options, they are inferior. This isn't necessarily the case, as there could be better options missed by the simplification of its decision routine, but unless you make the decision code more complex to allow it to see those better options, you would be making the AI do inferior things by letting it do anything other than its current first choice. So the problem is really that to give the AI choices that will result in consistent play at the level it currently plays at, you must first come up with a way to generate choices that are better than the ones it currently makes so you can use them as options.

It's also possible, though, that including some seemingly inferior options would actually help the AI by lending less predictability to it. In that case you are basically sacrificing one in favor of the other though, so you'd need a lot of studying to figure out just how much benefit is really gained by making it so the AI's moves aren't simply predictable.

There's another question too, even if the AI decision routine is improved: should you use that to make the AI have choices, or to simply improve the one decision it will always make? Again this ties into the predictability versus viability discussion.

Of course likely the best way to improve the entire situation you mentioned is to make the AI model better at predicting and reacting to threats. In the case you mentioned, the AI fails to employ the strategy you did because it doesn't recognize it is about to be railed by your crossbows. If it could realize that, it would avoid having it happen, and hopefully implement a better strategy. Similarly, its best recourse when you correctly predict its actions is to be able to fluidly react to how you counter it. It doesn't need a better strategy nearly as much as it needs better ability to adjust its strategy during the course of battle.