Dark Void

In "Into the Void" (episode I, chapter VI), after killing the Archon, your next objective will be to go through a tunnel. Well this tunnel starts with a long free fall where you simply have to let yourself fall for 10 seconds and wait until the achievement to pop before using your jetpack for the rest of the sequence.

It's later in the game as compared to "Spare parts" but it's safer since the tunnel is much wider than the previous free fall. No risk to run into a wall and die trying.

This achievement can also be picked up on the Episode II level "Breaking Camp": simply fly straight up in the air until you bump your head on the glass ceiling and start heading down; turn off the jetpack at this point and start freefalling. When you get really close to the clouds at the bottom of the Void, hit A to apply the brakes and stop the fall.

The achievement will unlock as soon as your descent stops.

For this you need to shut off your backpack and freefall for 10 seconds.

The best place to do this is Chapter 2-4 Breaking Camp. Fly to the top of the arc, and then boost straight up until you hit the end of the map and get turned around.

Set yourself to hover and move yourself so that there is no ground beneath you, and press B to shut off your pack and go into a free fall. Give yourself a 11 count (to be safe) and hit A to turn your pack back on. You need to survive so make sure you don't drop too far.