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.
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.