How to Beat No.12 of Mushroom XIII in Kingdom Hearts II

Locating Mushroom no.12

Go to the haunted mansion at Twilight Town. This is the same mansion where Roxas meets Diz and Namine, as well as the place where Sora awakens after Roxas' prologue.

Walk up to the main gate. Mushroom no.12 can be found right in front of the mansion, at the main gate just outside the mansion foyer.

Preparing Before the Challenge

Unequip all items with MP Haste. This will be items such as Ultima Weapon and Full Bloom+.

Unequip all of Sora's MP Haste abilities. You don't want Sora's MP to heal.

Equip all Combo Plus abilities. Unlike most of the mushroom challenges, no.12 will require you to have longer combos as you do not —absolutely do not—want to use finishers.

Equip Berserker Charge ability on Sora. This ability lets you attack continuously, and completely bypassing finishers. It only activates once Sora's MP is depleted (pink bar), and deactivates when Sora's MP regenerates (blue bar).

Equip Sleeping Lion Keyblade. It has the Combo Plus ability, which will let you do longer combos.

Challenging Mushroom no.12

Drive into Wisdom Form and cast Curaga. This is to deplete your MP to activate Berserk Charge. Press ∆ to start the challenge. You'll immediately see why this is hard if you panic while doing it.

Press R1 to lock-on a mushroom and shoot it. Now you just need to kill 39 more! The problem is the stage. There are too many things that can block your magic shots, and Wisdom Form has a problem with shooting things on high at close range.

Stay in the middle of the area. This is to avoid having to dash across when the mushroom appears on the other side. This will also help remedy the problem of pillars blocking your shooting range.

Go either left or right, depending on where mushrooms appear. Don't even try to cast Thundaga to eliminate the hard-to-reach mushrooms. It just won't work, and you'll waste precious seconds.

Quick Run away. If a mushroom appears on top of a pillar, Quick Run (analog + hold □) away from it. You won't be able to hit anything that’s a bit higher than you are unless you create some distance between you and your target.

Keep repeating everything. Just keep at it until you reach the goal. It shouldn't be too hard once you get used to it.