Also Check out: https://www.youtube.com/watch?v=U_qt_JUsmOY
BEGONE LAG YOU FOUL BEAST: https://www.exitlag.com/refer/443658
Quoted from NegaBioLizard : http://www.gamefaqs.com/boards/932981-final-fantasy-xv/74659761
"FYI everyone, here's how you get 500,000+ EXP before getting an EXP multiplier from a hotel. If you want to get a 3x EXP multiplier, you'll need access to the city of Altissia in Chapter 9 and the requisite 30,000 Gil.
1) You'll need to farm Rare Coins in the Cleigne region. Walk up to a sparkly blue treasure and save your game. If it's not a Rare Coin when you pick it up, reload the save and try again. Repeat this until you have at least 24 Rare Coins. (This could take 1-2 hours) You will also use your remaining Debased Coins, Silverpieces, and Banknotes.
2) Craft Expericast with a Lv. 96 EXP Bonus by combining any 1 magic (fire, blizzard, or thunder - no need to use 99, just 1 of any will suffice) with 8 Rare Coins. You will receive 5x Expericast spells to use. Do this two more times with your remaining 16 Rare Coins. Do the same with your remaining "Debased" items. You will not want Firaga/Blizzaga/Thundaga level spells because you will want to use as many spells as possible in a short time without killing a particular beast.
3) Visit the Wiz Chocobo Outpost and buy the Fat Chocobo Triple Decker sandwich for a 50% EXP boost. If you haven't unlocked it yet, just do the quests for the Wiz (the local "restaurant owner") until you complete "A Feathery Feast." (It's only the 3rd quest, I believe) The sandwich costs 1,100 Gil. Also, if you have the Moogle Charm, make sure to equip it for a further EXP boost.
4) Travel to the area just north of the Hammerhead and fight the Lv. 38 Bandersnatch. Unload ALL of your Expericast magic in one battle with him. If the Bandersnatch dies before you can use all your magic, you can get him to respawn by saving at the Hammerhead and reloading your save, then going back to his location. You should net your 500,000+ EXP once you've used all your Expericast spells, provided that your 50% EXP boost hasn't worn out. This is the shortest part of this whole ordeal, as you should be able to use all your magic in less than 10min.
5) Go to the hotel of your choice to cash in your EXP multiplier. Obviously, Altissia will provide the most. I went from Lv. 52 to Lv. 87 after doing this, and the added HP and strength are definitely worth it.
Good luck and happy hunting!"