How do i get the cube in level 18?

In the massive room with all the turrets and such?
Well first off destroy all the turrets, easiest way is to redirect energy balls at them. Next, you have to activate the moving platform. To do this, there's an energy ball redirect. To the left of the door you came in, there's a button on a raised platform. You have to press that, and then redirect an energy ball so that it goes into the receptor. The timing is really tricky, but you should get it after a couple tries. From there get on the lift and jump down to the little room.

Now THERE, there's a button that opens the door. After you press the button, portal back to the platform you start off on, and place a portal on the raised door. Then, jump off the edge of the platform to the right of the door and make a portal, which should fling you into the little area with the cube.