Can't beat Advanced 16 and 18.
In 16 I got to the part where there's the room with 3 turrets and a button, I used every cube from the malfunctioning dispenser to block the turrets, then I slowly crept over to the door, put a cube on the button, and was about to go through only to find there's another turret pointed at me on the other side. Ragequit.
For 18 I'm in the huge room with the turrets on stands. I knocked off all but the turret closest to the lift on the side you come in on. I can usually get passed it by crouching, then going from the back of the lift to the front when it notices me, but it's annoying, plus I can't nail the Portal jump from the slanted platform door to the small patch of ground on the other side to get to the cube.
When I did my original run-through I missed a single camera. :V