#23 was giving us fits here. I can honestly say that I wasn't really trying when it started to fall together. I was getting fed up and about to leave for lunch when putting a piece in a certain spot made things fall into place.
I'll give you a hint...if you want more or better hints, just let me know.
Hint: If you cross a light three times, it will be red. So, you are going to be crossing the middle light 3 times.
One more hint: The last two lights to get activated are the one at the bottom right of the bottom wall and then the top right one sitting just to the left of the bomb, and in that order.
One more: The first piece you need to use is going to be a mirror two squares right of the second red light when beginning. This will send the beam up to light a third light.