Kingdom Hearts 2 Final Mix - Mushroom 8

The mushroom everybody loves to hate. This is by far the most consistent strategy that I found. These are my ablities:
Sora
Horizontal Slash
Aerial Sprial
MP Haste
MP Hastera
Berserk Charge
Donald
Fantasia
Don't use any other MP Haste abilities or accessories that have the ability on them like Full Bloom+. I got my Berserk Charge ability from the Fatal Crest keyblade but alternatively you could also just equip the ability if you have it and use a keyblade with a longer reach like Guardian Soul instead.
The way I do it is by draining all of my MP, then waiting for the MP charge bar to reach the letter R in drive. Start the minigame and begin the Horizontal Slashes. When the mushroom gets launched into the air, use Aerial Sprial to reach it and continue the Horizontal Slashes. Around 70 hits is when your MP should be recharged with this setup, at which point you can use Comet. Comet itself does not add hits to the counter, but it does keep the mushroom in the air so you can get the last hits in. It's not 100% consistent. Sometimes the mushroom will still fall to the ground before you can get to 70 hits and there's not much you can do about that. It happens. But once you get to 70 you should be safe thanks to Comet.