copystuff

public void SortByAttack(List<Individual> original)
{
    List<Individual> muhUnitbyAtk = original.OrderBy(o =>o.GetAtk).ToList();
    original.Clear;
    //after this you want to copy the new ordered list into the original
    //there are two wasy that i found
    original = muhUnitbyAtk.ToList(); //I think you need another library for this one
    original.AddRange(muhUnitbyAtk); // this is the one I recommend



}