Do they mean by “plain magic” that the Prophet himself has fabricated the Qur’an. If so, tell them: “If I have fabricated the Qur’an, God is going to punish me and none of you can save me. God knows that your accusations are baseless and He will be judge between you and me. God is the Most Forgiving [so take advantage of it as the door is still open], the Most Merciful [that He still gives you time.]