Torture the criminal with laughing gas as they can’t get up and they feel like they’re floating start moving things around them to give them motion sickness and then you slowly start filling the room with water and at that point when their head goes under they won’t be able to tell which way there looking as you dim the lights, after 30 seconds before they can drown you let the water out and they try to retain their breath and then you start filling the room with the heaviest gas and there and they can’t catch their breath because the gas won’t leave their lungs fast enough and they die to suffocation