This can be obtained towards the end of level seven reunion. After the battle against the warden cortana will start speaking to you and you will venture into the mountains, she will tell you that you can proceed by foot or use a phaeton to go through. Simply go by foot (jump pads are fine) and the achievement is yours.
This can also be obtained on legendary
If in co-op all players must walk or the achievement won't unlock, thanks PTGigi for the info