Easy way to gain XP, especially in the late game.
How to Gain XP Easy
Before Grind Prep
Before being able to grind xp you need to have beaten the Final Fantasy 14 crossover quest. Before starting the quest I would recommend being at least level 30 and eat a food that buffs your stats at the nearby haven (anything you prefer). I would also recommend grinding Ignis to level 9 cooking.
The boss you will be facing is level 40 Garuda. You can access the fight using the white sphere drone in the fort where you did the FFXIV quest. If you feel confident enough then eat the lasagna that gives you 100% (2x) XP gain. During the fight I would recommend switching to Prompto and using the SMG while close to the boss. After running out of SMG ammo switch back to Noctis and start using the Ragnarok to warp-strike the boss (Ragnarok does extra warp-strike damage).
Eventually Garuda will summon 2 clones of herself which are weak to greatswords so use Gladio and his abilities to kill them easily. While the 2 clones are alive they will buff Garuda so it is advised to kill them ASAP. Most of the fight is just trying to learn her attacks and what to do at what times but eventually the boss will die, giving you TONS of xp. If your characters are level 100 you can kill Garuda in around 2 minutes with Promptos SMG. After beating her I think you earn around 200k xp which is really good if you want to level up early game.
After beating Garuda you will get an item which you can trade to Y’Jhimei (The cat person) for Vortex Weapons. Vortex weapons have very high stats and are often forgotten because no one does the FFXIV quest after they beat Garuda for their first time. First you must buy the weapon from Y’Jhimei using the item Garuda drops and a specific weapon (such as the Claymore for the Greatsword) and then you can upgrade it to a very powerful form using more of the material Garuda drops. Even with them being so powerful I would still recommend using the DLC weapons (Ones you get for beating character DLC’s) because you have them from the start.
Garuda fight gives hella XP and has good weapons, Prompto is very good during this fight because range.