By now I have satisfied every Mushroom except I and XIII.
Is there some special trick to No. I?
As the most hits I seem to be able to get in are around 50 to 55, nowhere near the required 70.
After that it's on to the Org Data Battles.
Is there a recommended order in which to tackle them?
I planned so far to start lefthand with Larxene and then go clockwise around the room.
What I did for Mushroom I was use Wisdom Form and attacked after I used Reversal. I usually waited about a split second after the command appears before hitting triangle.
As for the Organization members, I tried
to go from XIII on down to I, but when I got to Luxord and his timing mini-game at the end, I got so frustrated that I had to take a break from the game for a couple of weeks. I'd still recommend saving Xemnas for last since he not only has two fights instead of one, but he is also the most difficult of the Data battles by virtue of that one attack where he tries to steal Sora's heart and you are forced to play as Riku (I'd usually just spam Session once I'm about halfway down that boss fight). I beat all of them eventually, by going Roxas, Larxene, Marluxia, Saix, Demyx, Luxord, Axel, Zexion, Lexeaus, Vexen, Xaldin, Xigbar and finally Xemnas.