Mission 66: Restore the town's Heartless

Mission 66 in Kingdom Hearts 358/2 Days sends Roxas to solve the mysterious disappearance of the Heartless population in Halloween Town.

Regular mission
Restore the Town's Heartless by defeating the Leechgrave.

Story summary
Roxas is sent to Halloween Town to find out what happened to the Heartless population, which leads him to an encounter with the Leechgrave Heartless that seems to "eat" other Heartless.

Mission walkthrough
Set plenty of Fire panels prior to this mission, along with many Hi-Ethers. Entering the Halloween Town Square, Roxas sees where all the Heartless have gone. The town is now "infested" with Tentaclaws and Roxas must defeat all of them in the square before he can proceed. Quite soon, Roxas will figure out that wiping them out by using aerial combos takes a very long time. Casting Fire spells from a distance takes them down much more quickly, but conserving magic for the upcoming boss fight is more important.

Challenge Mission
Completing this mission may take a few tries, as you not only have to rapidly take out both swarms of Tentaclaws, but also deal with the Leechgrave itself. To take down the Leechgrave, you'll have to largely ignore the Tentaclaws it summons and just go straight for the main boss. Limit Breaks work well here, and Auto-Life is a must. Dodge the main body's attacks while dealing damage of your own, and you should be able to achieve victory. If you expanded the break in the wall by using another bomb in the mission where you fight Lock, Shock, and Barrel, you can save some time by going straight through it.

Like the timed mission, this one is a nuisance because you have to deal with three waves of enemies. Survival is key, so you want to have plenty of Fire magic and be proficient with Guard. When you get to the main boss, you have to not only dodge the standard attacks, but also the poison clouds that the main body will create, as getting damaged by them counts as taking a hit. Take your time, and have plenty of Elixir's handy to restore your magic. Another way to do it is to equip the Extreme ring, but you will need the Auto-Life panel. This will give you permanent access to Limit Breaks. Take out the Tentaclaws with Pumpkin bombs or Fire magic from a distance, always keeping an eye on Leechgrave. When Leechgave falls down, hammer it with a Limit Break until either it gets up again or it is defeated.