Here are two options.
First: The fight will take place right on the map. Button for attack will be for example X or C.
Second: Battle System remains original of the RMXP ... with one difference. There will be a gunsight that will be controlled by the mouse. The gunsight will slightly move to not focus too easy. When you hit head, the enemy dies. When hit the body, enemy's HP will be reduced (in the case of zombie nothing happens).
Script must be very easy to use or i will not understand it.
In the case of the second battle system, it will be necessary to create a "Battle characters"! But even so, the second option sounds more interesting, right?