Provided you start the game on Normal, this achievement is story related and cannot be missed. The difficulty mode cannot be changed once the game begins so the only condition that must be met is to finish the game.
Just before the end of the game after the final boss battles, Ardyn is provided with a choice. Either answer is acceptable. Once the selection is made, cut scenes pertaining to the choice play out. The achievement unlocks following these cut scenes.
The achievement is not difficult because there are numerous save opportunities marked by the campsite icon in the City of Insomnia and, before that, in the lab. There is a save point outside the gates just before the final boss battles.
Items can be bought in Insomnia to increase power, defense, magic, and spirit. Curatives like Elixirs to restore all HP and MP can also be purchased. These assist in all fights and are especially useful in the final boss battles. Ifrit can be summoned to fight alongside Ardyn. His initial appearance does great damage especially if the three-bar summon, Meteor Strike, is used. In other words, with the resources available to you, this is not a difficult completion even though its played on the harder difficulty.