Portal: Still Alive

Or you can go to Dmbledore's site, where he has video guides for every challenge, and unlike most video guides, these were done while playing on the 360 and not the PC:

Story: https://www.youtube.com/playlist?list=PL3E390000FC5DD679

Advanced Chambers: https://www.youtube.com/playlist?list=PL1471E80DCBE90F73

Credit obviously goes to Dmbledore himself.

edit: In the comments, Slider257 points to a better/easier solution to the final advanced chamber. I had put that video into this solution but it would appear it now has gone down.

There are levels 13-18 with advanced maps(6) that only unlock AFTER you beat the game. Thought this would be good information to know instead of searching everywhere.

This achievement requires you to beat the 14 challenge maps, and a further 6 advanced maps that were in the orange box version.

For a good video guide on how to beat the advanced maps go to:
http://www.portalwalkthroughs.com/

For a written walkthrough (with pictures) of the challenge maps go to:
http://uk.guides.ign.com/guides/14266556/page_32.html

There does not seem to be one website containing a video walkthrough of the challenge maps, but sickder666 has many good videos of how to beat them on youtube, and i must say i have been using them thus far and i find them very helpful.
To view his videos go to http://www.youtube.com/profile?user=Sickeder666&view=vid...

The last test chamber is a pain, even with video guides, due to the insane jumping puzzle at the end. The easiest way to get by this is by abusing the save system. Every time you hit a platform, quickly hit the start button and save your game in a new slot. That way you are making the exact same moves every time. It's also a lot less frustrating since it feels like progress.

To go along with the other solutions and videos, I've included a written guide for the jumping bit below. The hardest part is getting to know where the platforms are located.

- Shoot an orange portal in the middle of the first island.

- Shoot a blue portal in the far corner of the pit.

- Jump in the pit. Make sure you jump so you get the extra momentum. You want to jump in the far corner, which will help guide you straight into the portal. NEVER TOUCH THE LEFT STICK AFTER THIS POINT. You will likely miss a portal since Chell has Mario physics and can steer in the air.

- The first platform will be right in front of you. Shoot a blue portal at it.

- The next platform will be almost behind you. Shoot an orange portal at it.

- The next platform will be a little to the left. Shoot a blue portal at it.

- The final angled platform will be a little to the right. Shoot an orange portal at it and get launched to the exit.

- Enjoy your achievement.

Hope that helps!