You get this achievement by completing Chapter 13;
Probably the hardest and worst chapter in the entire game but thankfully it now has an alternative, after the
March 2017 Update:
*** Spoiler - click to reveal *** You can now play this chapter as Gladiolus in the what is called Chapter 13 Verse 2;
At the beginning of the chapter you will be able to choose if you want to continue as Noctis or play as Gladio, if you decide to go as Noctis and regret halfway through (like I did...) you can change to Gladio at one of the resting/save rooms that you will find on your playthrough; It's worth noting that going back from Gladio to Noctis is not possible but honestly, I don't think you will be missing much...
Playing as Gladio is way easier as you get to skip several areas that Noctis would have to go through and it's more combat focused gameplay instead of the stealth/puzzle focus gameplay that you would have to go through playing as Noctis.
Verse 2 will take about 20-30 minutes to complete and you'll get this achievement the same way you would doing the regular run;