How to implement the Visitor pattern for subtypes of an abstract class or interface?

I'm having this specific issue I couldn't sort out with other questions. I'm trying to implement a Visitor for a game, the visitor is an Attack class and it has to search in a matrix for cells that may contain Characters, then if the character is an enemy and not a friend, damage it.

What I'm having trouble is not using an InstanceOf to visit the character, as it breaks the Open-Closed Principle. Here's my code:

Visitor interface

public interface Visitor {
    public void visit(GroundCell c);
    public void visit(MountainCell c);
    public void visit(BuildingCell c);
    public void visit(WaterCell c);
    public void visit(Foe f);
    public void visit(Friend f);}

Attack abstract class

public abstract class Attack implements Visitor {

}

Attack concrete class

public class TankAttack extends Attack{

...

@Override
public void visit(GroundCell c) {
    //here, i'd like to call  c.getCharacter.accept(this) 
}

But I get an error that says I should first implement Visit(Character c), when I need it only for its subclasses.

What should I do in the TankAttack class to visit the Friend or Foe subclasses of Character and not break the design using InstanceOf?

Edit for clarification: Friend and Foe are subclasses of Character.