This should unlock upon completion of level 24: Observatory.
It does not seem to matter whether you pass or fail. I failed just to test and the achievement still unlocked.
If you're having trouble, take your time using less power to transfer from platform to platform. You can get to the second platform with the portal and key with one medium power shot, and then skip the third platform to land on the final platform with a fully powered shot.
There is also a "hidden" key off the ramp of the second platform (with the purple portal).