Towards the end of the game you will be teleported into the skies above Kazakh, tasked with boarding the Balrog airship to destroy the third Gravitron. You start off on your approach, and have to make it from the start of the level to the Balrogs deck without falling. If you do fall, then once you have respawned let an enemy kill you. This returns you to your last checkpoint and you are free to try again. You can also quit to the main menu then reload your save if you prefer. Be warned, this is the second 'truly' missable achievement. If you leave this level, you may never return to try again without starting a new game.
Just going to post this because i wish it had been here when I needed it. To get this achievement, all you need to do is navigate your way TO the Balrog without falling off. To clarify further, this means when you're jumping from air skiff to air skiff, don't fall. The second your feet touch the ships deck, the achievement should pop.