God paradox
One answer is that God could make it so he can't lift the mountain by his own choice. In other words, God can lift the unliftable mountain because he is all powerful, but he chooses not to be able to, and so he chooses to cut his own power, because he is able. God can also choose to not be able to create such a mountain. God is all powerful, so he can choose not to have the power to make an unliftable mountain. Simply said, God can cut his own power, but in so doing, he can still regain that power. It is a limbo, a state between not powerful and powerful, something that is not definable by our knowledge of what is possible or impossible. Because God is almighty, he surpasses our definitions of what he can and cannot do.