Decided i would make an updated version of my old tutorial of this.
How to make a falling floor in minecraft.
They will fall down through a hole and die.
This falling floor technique was tested in mine.
First and probably the simplest set up a room that is 2 wide with a trapped chest at the end a trapped chest will give off a redstone output when opened so hook it up to a redstone torch to invert the signal and then put sticky pistons that will retract the floor.
Insert random nonsense here.