What is the ruling on supplicating against a disbeliever for lack of mercy by saying: "O Allah, do not have mercy on him"?

A living disbeliever should be prayed for guidance, and one should not pray for him to be denied mercy due to ignorance of his ultimate fate. It is permissible to curse disbelievers in general, but regarding cursing a specific disbeliever, there are two opinions, and it is better to refrain from it due to the possibility of his repentance. The supplication of Noah (peace be upon him) against the disbelievers, and the supplication of the Prophet (peace and blessings be upon him) against those who allied against the believers, are a basis for supplicating against disbelievers in general. However, a specific disbeliever whose end is unknown should not be prayed against, because his ultimate fate is unknown.