18 - stack all the squares up the middle, then with each of the grey bars, place them on the edge of the circle so they lean in towards the tower of squares.
20 - Place the circles down, with the grey bars on top. To make the tower more stable however, be sneaky and place the circles under an already placed grey bar.
Come on unholy knight, retro style games (including stacking) are what you DO brag about. Everyone knows retro games are harder to master.
Being good at Tetris > Counterstrike
Numerous people having trouble with #15, so here it is: Shoot a portal above the door you came in from, then another in front of the clear blue window. Jump in the second portal, come out the first, then try to land in the second again, thus propelling you over window, and just before the electric floors. Now shoot at your feet, and you should come out the portal above the door, then create a portal in front of the clear blue window (should replace the one above the door). Jump into this, which should make you go high enough to land onto the electric floors (hopefully when they're NOT active =P). The rest is all in the space bar.
The incompetence of the people commenting here is remarkable. I like the guide for the gray planet. "hit center". What the hell is center!? Better yet, you keep copying and pasting it. Again, I HAVE NO CENTER KEY.
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