Iron Grip Achievement
Never lose a cube in Chamber 6 of the Mass and Velocity co-op course
This can be gained in Course 2 – Mass and Velocity Test Chamber 6 and can be gained in either split-screen or over XBL.
This chamber is made up of 2 rooms. The first has a cube in it but you can't/shouldn't lose it. The next room is where timing is everything. Place a portal on the hanging slab above the entrance and then another on the incline one across the water to your side.
Now one of you use the launch pad to access the other side of the area and then get ready to use the launch pad next to the cube chute. Your partner should be at the start, ready to push the button to release the cube. What happens is, when its released it'll fall out and get sprung to the other end of the room near inclined portal but it won't go through it. That's the point when its that close that you have to grab it otherwise it'll be to high.
Hit the launch pad, and as you get about half way in the air, have your partner hit the switch. By the time you come out of the second portal, be ready to grab the cube right away. This all comes down to timing, so if you miss it and it falls into the water or its too high up for you to grab, then you can simply restart the chamber later on from the menu.