Which has not been held to be cruel and unusual punishment?

I believe the answer is: Execution by firing squad.

Execution by firing squad would use around 10 Gunmen that are told to shoot at the vital area of the convicts at the same time.

From those 10 gunmen, only three of them are filled with bullet. With this type of execution, the death of the convict would be quick and the executioner could do it with less remorse.