Thanks for the play and the helpful feedback!
You're right, the AI was tough to balance. Whenever I made a slight gameplay change, it made the AI either too easy or impossible to beat, so I had to rebalance often. The way I implemented it was that the AI picked a target spot to defend, but only had a limited reaction speed, like humans, and sometimes was inaccurate. Then I increased the difficulty by reducing the size of the errors, and increasing the reaction time.