There are two ways to get out of this mess: first, prove you are not infringing on their rights. That would mean that you prove none of the claims in their patent fits to you, or it is already covered by prior art. Thus, their patent would not allow them to infringe on your rights, as you aren't enfringing on theirs. Second, prove that the monopoly should not have been granted to begin with. Then there is no case, as there is no patent at all.