The guillotine worked every time.
That said, this wasn't cruel and unusual punishment. It was a mistake of approved method without intent.
Also, this was a meth head who murdered two people in cold blood. That he suffered for a whole two hours before shuffling off is not enough to trip the meter.
Firing squad seems most effective, predictable, and cheap. I'm not sure why it was abolished, but two states still allow it. It isn't cruel, the person usually dies immediately, and it is cheaper. Much cheaper.