No, God cannot create a grade so high that he cannot achieve it. God cannot do anything that is by definition impossible. For instance, what happens when an unstoppable force meets an unmovable object? Answer? Nothing... the presence of an unstoppable force negates the existence of an unmovable object. For the same reason, an all powerful God cannot create an object so big he can't lift it, because that would make him, by definition, no longer all powerful.