Hello there are 2 secret levels i didnt talk about.
Chapter 7: 7-6
do the level normally. when you activate the pressure plates, freeing the sawblade, and go to the portal, go to the right again and theres gonna be a entrance to the secret level
the secret level uses the random teleporting gimmick
Chapter 8: 8-3
do the level normally, again. at the end of the part with volts, go throught the wall and you get to the secret level. its one of the annoying ones.