Blue Dragon

I actually found the Ancient Forest - Ruins to be a lot faster than the Golden Poo method, especially if you have at least a couple of Leis.

When you enter the forest there are 4 groups of Zebra Poos. Fighting 2 of these groups at a time nets just over 2,000 EXP and with the Lei equipped, that'll be just over 3,000 EXP. Since there are 4 groups you can net over 4,000 EXP without the Lei or over 6,000 EXP with the Lei in just a couple of minutes. Once you dispatched all 4 Zebra Poos just leave the forest and re-enter. I timed myself and I defeated all 4 groups and left the forest in 2 minutes flat. Having Mow Down 2 is essential to make this quicker.

Considering you need just over 12,000 EXP to level after around Level 80 or so, it shouldn't take that long to hit 99 and thus is the faster method I believe.

The fastest way i found to do this was to head to the southern deserted island where the golden poo snake spawns, equip dog collar for the stink effect aswell as shoes of hermes and poo bracelet for faster kills.

Lei helps get to lvl99 faster as gives extra 50% exp and just keep killing it giving 1010 exp each time.

Also each kill gives 10sp and a random amount of gold between 0 and 40000 so can help towards the class achievements and gold achievement at the same time

As described by others, go to the Southern Deserted Island (near Jibral Mountains, with the little green plateau in the middle of the island). Equip a character with the shoes of hermes, the poo bracelet. Equip all the Lei's you can, same with Black Belts (so you can level the Shadows). Eat some garlic or use the dog collar.

From the menu, turn the music all the way off, so that you only hear sound effects. Switch the inputs on your tv to something else (leave the xbox audio on and just watch regular tv, like a sporting event). Listen for the little "discovery noise" the poo snake makes when it spots you. When your controller vibrates, the poo snake has charged you to start a fight. When you hear it twirl its spear, just start pressing the "A" button. That'll make the character with the poo bracelet and Shoes of Hermes attack the one golden poo and kill it with one shot. Then after the tally noise, just hit the A button a few more times until it clears the results screen (including any new skills). When you hear the ocean, you're back to waiting for the next fight.

If you go a while without a fight, flip back over to see what's happening. Usually, a poo has gotten stuck running towards you - on the treasure chest or on trees or a corner of the plateau. Check the menu every once in a while to see if you need to flip shadows over to another type, avoiding guardian for as long as you can (since you'll get hearts for it - see below). With black belts and the eyepatch, you get 20SP per kill, so post level 60, a shadow class takes about 5 kills to go to the next level.

Generally you run around 1100EXP per fight; it's a long haul to 99. Note that Leis don't stack, so don't bother equipping more than one on a character.

Optional:
1) If you're looking to get gold too (for the 1 million gold achievement), use the ghost king's crown too, and gold enhancing accessories.
2) equip the poo bracelet character with the "Loot" and "Treasure Hunt" skills. The rare item for a Golden Poo's is a Guardian Heart, which you can use to help level a shadow to 98 (don't ever use a heart to go to level 99). The amount of SP needed per level tops out at lvl60, so use hearts anytime after that.

If you're like me and you finished this game in its entirity with plenty of leveling and shadows still to do, but DON'T want to do the grind yourself, and you have a turbo controller you're in luck!

1. Head to the SOUTHWESTERN Deserted Island. This is where the platinum poos and the large groups of other poos spawn.

2. Add Stink status to a party member or 2. (I just equipped a dog collar on a couple as one of their 3 accessories)

3. Park your ship at the easternmost end of the island behind the mountain. Head to the opposite end of the island, to the end of one of the smaller two peninsulas that stick out to the sides. It doesn't matter which one other than I would suggest NOT standing near the treasure chest area as enemies may get stuck behind the treasure chest.

4. Plant yourself on the shoreline at the end of the peninsula with your back to the ocean.

5. Make sure ALL characters have the following skills: Skill + 8, Accessory + 3, Absorb HP, Ninja Swiftness, and Double Strike (probably not necessary but speeds things up if you can't one shot enemies in a single attack)

6. Add Loot and Treasure Hunt to steal items you can sell for the 1,000,000 gold achievement (2 birds with one stone) if you like. FYI I earned over 1,000,000 gold while turboing my levels, didn't have to sell a single item, but I did steal max quantaties of several different items.

7. The following skills are optional but they may help if your characters are lower level and/or have low defense/agility: Guardian Shield, Absolute Counterattack, Critical Hit Lift. Try to ensure everyone's agility is over 200. This isn't absolutely necessary but I highly advise it. .

8. Accessories: Dog collar someone for stink, belt of hermes and any other agility raising items, and equip any Lei(s) you might have for increased experience on a character or two. You can come back after 4 or 5 hours and swap the Lei(s) onto different characters to even them out.

9. You can also give each char a black belt and set them to a shadow they haven't maxed yet to work on maxing all shadows, although once they all hit lvl 99 (character level) then afking with lvl 3 field barrier in laser field is still the best way to lvl shadows while afk.

10. Turbo the A button on your turbo controller (tape it down or whatever method you prefer) and leave it running. I'm currently levelling from 52 to 99 doing this, and am at 75 so far without ANY issues at all.

I would still suggest the Laser Field + Field Barrier 3 method for levelling shadows once the characters are all level 99 as the enemies spawn faster and closer to you there, but for levelling characters to 99 without having to play at all this works great.