Q:

How to call a method from a specific ancestor class in Ruby?

Say, I have such a hierarchy:
class A
  def some_method
    'From A'
  end
end

class B < A
  def some_method
    'From B'
  end
end

class C < B
  def some_method
    # what's here to receive 'From A' ?
  end
end

c = C.new
c.some_method # get 'From A'

If I call super in C#some_method I'll receive 'From B'.
How should I implement C#some_method to get 'From A' in c.some_method. 
What is the best practice to do this?

A:

You can use "unbound methods" for this:
class A
  def some_method
    'From A'
  end
end

class B < A
  def some_method
    'From B'
  end
end

class C < B
  def some_method
    A.instance_method(:some_method).bind(self).call
  end
end

c = C.new
c.some_method # get 'From A'

Ruby has the capability to unbind a method from an object and then bind it to another object. The instance_method is used to grab a method object from the class rather than from a specific instance of this class. Later, we can bind that method to the instance of C that is calling some_method, that's it self, to finally call that method immediately in the same line.
As another user said, if your are doing this, you should probably review the desing of your program to use composition or another approach.

