Keeping Track Of Strategy
In your player class, make a variable that stores your team's current plan. The simplest option here is to use a String, but if you're comfortable with the syntax an enumeration is even better.
In the strategy method, you change this variable based on the state of the game. To start, simply start with a label like defense or guard.
Reminder: Use Your Unit Superclass
We're going to be adding a lot of code to handle this behavior within units. I strongly suggest you write helper methods in your main unit class. Beyond that, remember that your unit superclass has its own action method. You can use inheritance to have each subclass call super.action() to handle general behaviors before they peform unit-specific actions.