There's not much to do to really speed up leveling before the post-game. 5th Stratum gives you Blue Pasarans, though, so you can do the Red Pasaran grinding "trick" with them, though I find catching the 5th Stratum ones to be more effort than it's worth.
There's also the [hide]Scylla[/hide] exploit, assuming you've found Aeaea. It's not terribly fast (28000 EXP per fight for one character at a time) but it's effortless.
Red Pasaran grinding trick:
[spoiler]Set-up: Massive amount of buffs (Fore Honor, Attack Order, etc.) and ideally the ability to nearly one-shot a Red Pasaran. Bloodstained Spear helps a lot. You'll also want a Shogun _without_ Swashbuckling; it's not pivotal but it makes it far more mangeable without all those extra hits. That Shogun will also need one Ice-forged weapon and one Fire-forged weapon.
Basically, turn one you attack with the Shogun and spawn two more Pasarans: A Blue one (from the Ice weapon) and a Red one (from the Fire weapon). You need to do this _every_ turn as you'll be killing one Red per turn and one Blue will self-destruct per turn (typically).
Once you have that set-up, you merely ... kill the Red Pasarans as they spawn but before they escape. After 17, the EXP caps so you just finish the fight and repeat the process.[/spoiler]
[hide]Scylla[/hide] exploit
[spoiler]Save and then soft-reset. Choose the first fight against Scylla (1x Wilding, 1x YourCharacter; the quest is titled "For a Mutated Girl") and do not Save. It's not critical that you don't Save but it helps you know which step you're on.
1st Turn: Escape, Wildling summons Tiger and insta-kills Scylla.
Rinse, repeat.[/spoiler]
Edit: Sea Directions, courtesy of ThatSageGuy from GameFAQs [Topic:
http://www.gamefaqs.com/boards/981313-e ... y/56853450]
[spoiler]Directions: I have used these directions to get everywhere for the first time and I have double checked them, so they do work.
Lighthouse - Requires 16+ turn food. (Reward: Foremast, 500en) From Armoroad go west 3, south 1, west 7, north 2, east 1.
Batavia - Requires 16+ turn food, Foremast. (Reward: Sauerkraut, 700en) From Armoroad go east 2, south 1 onto the current, south 1, east 1, south 1, west 2, north 1.
Ayutthaya - Requires 16+ turn food, Foremast. (Reward: Ship Upgrade 1 (received later), 700en) From Armoroad go east 4, north 1, east 2, north 1, east 1, north 1, west 1, north 2, west 1.
Tortuga - Requires 20+ turn food, Foremast. (Reward: Cannon, 1200en) From Armoroad go west 3, south 3, west 2, south 1, east 1, west 2, south 2, west 1, south 1, west 1.
Ruins of the Giant - Requires 20+ turn food, Foremast, Cannon. (Reward: Iron Navel Ram, 2000en) From Armoroad go east 1, south 5, west 1, south 1, fire the cannon at the pirates, south 1, east 1 (ride the current), east 3, south 1, north 2, west 1.
2nd Foothold Route 1 - Requires 20+ turn food, Foremast, Cannon. From Armoroad go east 2, south 1 (onto the current), north 1, east 2, north 2, west 1, north 2, west 1, north 1, west 2, north 1, fire the cannon at the pirates, north 3.
2nd Foothold Route 2 - Requires 20+ turn food, Foremast, Iron Navel Ram. From Armoroad go north 4, east 1, north 4, east 1.
Dark Forest - Requires 8+ turn food, Iron Navel Ram. (Reward: Steel Plating (to be received later) 1800en) From the 2nd foothold go west 5, north 3.
Sheba Route 1 - Requires 20+ Turn food, Foremast. (Reward: Salted Meat, 1500en) From 2nd Foothold gowest 1, south 1, east 2, north 5, east 2, south 2, west 1, east 1, south 1, east 1, west 1.
Sheba Route 2 - Requires 8+ turn food, Iron Navel Ram. From 2nd Foothold go east 2, north 2, east 3.
3rd Foothold - Requires 24+ turn food, Foremast, Steel Plating. From the 2nd Foothold go east 1, north 4, east 1, north 2, east 2, north 1, east 2, north 3, west 1, north 1, west 1, north 1, east 1.
Damavand - Requires 24+ turn food, Steel Plating. (Reward: Ship Upgrade 2, 6000en) From the 3rd Foothold go west 2, north 1, west 5, south 3, west 3, south 2, east 1, south 3, west 4.
Tower of Victory - Requires 24+ turn food, Foremast, Iron Navel Ram, Steel Plating. (Reward: Latin Sail (to be received later), 8000en) From the 3rd Foothold go west 2, south 5, west 2, south 1, west 2, south 3, west 5, south 1.
4th Foothold - Requires 20+ turn food, Latin Sail, Iron Navel Ram, Steel Plating. From the 3rd Foothold go west 1, south 4, west 2, south 1, west 1, south 2, north 1, west 1, south 1, west 4, south 2.
Aeaea - Requires 24+ turn food, Latin Sail, Iron Navel Ram. (Reward: Carronade Cannon) From the 4th Foothold go north 1, east 1, ( ), east 1, north 1, east 1, north 5, west 4, east 1, north 2.
Ugarit - Requires 24+ turn food, Cannon. (Reward: Elephant Turtle) From the 4th Foothold go north 2, east 3, shoot the cannon at the pirates, east 1, south 2, west 1, south 8, west 6.
Floating Island - Requires 24+ turn food. From the 4th Foothold go west 3, north 4, west 1, north 1, west 1, north 2, south 1, north 1, east 1 (onto the current), north 4, east 2, east 1, south 1, east 1.[/spoiler]
