Final Fantasy XV has an unique level up system that takes inspiration from the Dark Souls series. The experience points are given after each fight but they aren’t used until the player manually choose to either rest at the various camping points on the world map, or uses an inn/motel at outpost.
Some of these locations actually have an additional benefit of providing a good chunk of experience boost once we rest there. While camping doesn’t provide any benefit aside from free cooking from Ignis, which helps with stat buffs, there are some locations that require a lot of gil in order to provide a boost when utilizing them. There is Galdin Quay in the game which can provide 2x the experience points gained, although it costs an expensive 10k, but there is actually a far more prominent location later in the game, which provides 3x the experience points gained.
To access this rest point, you will need to progress through the story until Chapter 9, when the party reaches Altissia. The rest point is located there and provides 3x experience points that the player has accumulated throughout his journey. If you want to easily level up to 99, you can keep the experience points you gain by doing hunts and sidequests until you rest at the location in Altissia.
Experience points are gained by finishing the story missions, side quests, hunts, and fighting the monsters and niflheim troops. There is also a beast whistle available later in the game that summons monsters to fight and to properly utilize it, we recommend to head into a dungeon with monsters having an average of level 40-50 and summon them using the whistle. This will result in easy experience points without the hassle of travelling all over the world map.
Final Fantasy XV is available now for the PS4 and Xbox One.