Walk on Air - 20G (Original Achievement)
To get this Achievement, you must clear Sky Sanctuary, Act 1 without falling to your death.
The best method is to take your time and avoid as many wide open areas as possible. Be cautious on the green block platforms, as they fall from under your feet if you stand on them too long.
Below is a video of mine showing how to clear this, it may not help but it should give others a better understanding of the level.