Chapter 10 - Testchamber 18
This one will probably give you vertigo by the end. Start out by look across the hallway. Put one on the ceiling above the first platform and another portal on the wall to the left. Jump through it to cross the gap. Look above the gap and shoot another portal above the second platform and one in the wall in front of you. Aim low and jump through it. Repeat this for the third platform, except you can just use a portal in the floor. In order to get over to the platform with the button, aim above at the panels and get ready for a slingshot. Put a portal on the pit below and slingshot the gap.
Go past the button and push the little red button to gain access to the well defend chamber. There’s a box dropper across the way and the only way to get there is to jump off the right side for a slingshot while the entrance is still angled up (for maximum launch). The door closes quickly, so this is a little complicated. We also need the tram running to be able to get the cube back to the entrance.
There are turrets everywhere, so lets get rid of them. Just put one portal on where their laser point ends and another portal at the burned spot, hopefully right before the moving launcher fires the ball (to save time). Once the turrets are down, look to the left. The receiver is behind a shield. We need to get an energy ball to head toward it and then do a quick portal setup to hit the raised button.
Aim across the toxic pit and hit the panel across from the receiver. As soon as the ball goes through the portal, shoot one at the base of the button and one at the outstretched panel beside the button. Jump through the portal and smash the button. You should be able to do it with a second or two to spare. The energy ball will hit the receiver and turn the tram on.
Ride the tram over and jump down to reach the little inlet. Press the button to open the entrance back up. Put one portal next to it and another portal on either of the white walls by the button. Jump through it to get back to the entrance. Put one portal on the diagonal panel and leap off of the side to slingshot all the way over to the block. Grab it and jump onto the tram and run the block back over to the other side.
Repeat the trick to reopen the entrance and carry the cube back through to the first area. Drop it on the button and cross the gap to get to through the door. Walk through and look out. We’ve got a very complicated trick to pull off.
You need to alternate the portals and move up through the platforms and then slingshot across. You can’t move up to the next platform with forward momentum, so you have to keep track of the portals and shoot a new one with each jump.
To start with, put a blue one on the flat platform in the toxic water (again, color doesn’t matter but you have to remember which ones you used). Use an orange one at the bottom of the pit and jump into it. After you go through and fly up, look down and fire an orange portal onto the next platform. On the next trip, you just have to do it again. Look down and to the left. After that, the last one is a small diagonal panel in the back corner. Hit it with the last portal to get flung across the gap and reach the elevator.