Right, I've hit the achievement, and here's the info:
I've now counted 16 monitors in Chapter 8, but you can ignore the first 5, so you need to break all the monitors from the one found in the test chamber numbered
1/19, up to the start of Chapter 9.
Instruction for breaking each monitor:
Move the cube to the lower section of the level. Use a ceiling-floor loop to get the cube to high speed then fling it from the opposite wall to the monitor.
Move a cube to the ceiling using the vertical funnel position that forms the normal solve of the level and drop the funnel to fling the cube at the monitor from the wall in front of it.
Another cube fling, either using a ceiling-floor loop or funnel to get the cube up to speed for the fling.
You need to sneak round to behind the turret (I managed to run to the wall next to it before it could kill me, wait and then run again) and carry the turret towards the monitor. As long as you do this with the turret being "OK" (ie, don't let it shut-down) then it will target and shoot the monitor for you. You might have to put it down (as long as it lands correctly).
Chamber 5 (has 2 monitors)
The closest monitor is broken by flinging a cube from the opposite wall using a vertical funnel in the centre. Place the launching portal directly under where the funnel hits the wall to ensure you're on target.
The second monitor only appears when you reach the exit door, and is broken by the same method but by flinging the cube from the far-right space on the angled launch pad opposite.
Just step on the aerial faith plate to launch yourself at the monitor.
Once you've set up your laser (for completing the level) instead place the exit portal on the wall facing the monitor near the target point for the laser. Then maybe tweak the angle of the laser with the box to point it at the monitor for a few seconds.
When you're setting up the laser to aim at it's target, just point it right a bit (at the monitor) for a few seconds.
I did this with a double-fling of myself. Set a on the opposite wall and one of the floor below it, step in to the one on the floor then fall in to the one on the floor again - this second fall should give you the speed to fling across for the break.
This one was quite difficult, but basically you have to get hold of a turret and get that to shoot the monitor (as per chamber 4).
First, I used blue gel to remove two of the turrets (instead of all three as you would to solve the chamber). Then I carefully placed a portal on only half of where the funnel is hitting the wall and linked it to one on the floor - this means the funnel doesn't "pass through" the portal, so you can effectively sit on that wall. From here you can shoot the other portal in to the wall above the turrets, then reposition the portal that's misaligned on the funnel's wall to let the funnel through. Then you can ride the funnel and drop down on top of the turret.
Once you've started chapter 9, when Wheatley tells you that you're getting close to his lair, you can blow up the monitor using the falling bombs (this is the room that he tries to kill you with the spike plate and conveyor belt).
Please note that this does not contribute to the achievement.