There is a side mission that becomes available in Chapter 8, where you have to defeat Formouth Garrison (which is also the location of a piece for the flying Regalia Type-F car and a special upgradeable weapon). You can find the fort in the far north-east corner of Leide, opposite of the Entethina Haven camp. You can visit the fort at any point, but you cannot begin the assault without the side mission that becomes available in Chapter 8, after taking down the previous two forts. Just get in close proximity for the quest to begin. Make sure your party is fully healed and you have plenty of magic. Have the strongest spells you can make and a full 99 of each element. Make sure you also leave 1-2 Magic Flasks free. Set off after you camp. You can use any of the foods that provide XP bonuses, but the Lasagne Al Forno recipe provides a 4,000 HP, nullifies all elemental damage, and gives a 100% bonus to all experience points. This is unlocked through cooking with Ignis. Once you are buffed up, create some Thunder spells using the Expericast buff items. These include Debased Coin, Debased Silverpiece, Debased Banknote, and a few other items. Combine these, using as many as possible, to create a couple of Expericast spells with 99 potency. Use these throughout the quest, especially on the last two Boss fights for massive experience. At the end of the quest, do not leave. Turn around and explore the area to find more Boss battles with giant mechs. Combine the experience inside to get nearly 500,000 experience. Then, go south-east once you get outside to find another huge robot dropped by an airship and some great experience points. Proceed to Galdin Quay and stay at the inn there for 10,000 Gil to double the experience to the nearly 1,000,000.