Selecting a script from a Gameobject[] in Unity

Assuming you're using inheritance or interfaces, you can create an array of your base class/interface, and add all your creatures into that array (which you say you have an array of all these creatures).

You would simply loop through all the elements in the array, and call the respective method.

For example:

//not a cut-and-paste example, but gives you the general idea.
foreach(var _creature in Array)
{
   _creature.Die();
}

However, if you are trying to call a different method for each one, you may find this will not work for you.

If you want to break it down further, you can try and switch on the type and then call a specific method for each type, but then it isn't a truly generic class:

For example:

//not a cut-and-paste example, but gives you the general idea.
foreach(var _creature in Array)
{
   switch typeof(_creature):
      case LowLevel:
        _creature.Flee();
        break;
      case MidLevel:
        _creature.Guard();
        break;
      case HighLevel:
        _creature.Attack();
        break;
      default:
        _creature.Idle();
        break;
}

An example of a polymorphic implementation:

public abstract class Combatant : MonoBehaviour
{
    //for arguments sake, we are only going to have speed property, attack method which
    //all instances have to implement, and a defend which only some might implement their
    //own method for (the rest default to the base class)
    int speed {get;set;}

    //Need to have this method in derived classes
    //We don't define a body, because the derived class needs to define it's own
    protected abstract void Attack();

    //Can be overriden in derived class, but if not, falls back to this implementation
    protected virtual void Defend()
    {
        speed *= 0.5; //half speed when defending
    }
}

public class MonsterA : Combatant
{
    protected override void Attack()
    {
        //attack code goes here for MonsterA
    }

    //to access speed, we simply need to use base.speed
    public void SomethingElse()
    {
        base.speed *= 1.25;
    }

    //notice we don't have to provide an implementation for Defend, but we can still access it by calling base.Defend(); - this will then run the base classes implementation of it.
}

public class MonsterB : Combatant
{
    protected override void Attack()
    {
        //attack code goes here for MonsterB
    }

    //Assume this is a 'heavier' class 'monster', speed will go down more than standard when defending
    protected override void Defend()
    {
        base.speed *= 0.3;
    }
}