Mushroom 10, 12 for now. I won against 1, 6 and one or two others. Haven't met the others except 8.
The others are piss easy if you follow a guide. 8 is the tricky one, but one of Donald's Limits makes it manageable.
Is there a guide online somewhere on how I should approach post-game contents for KHIIFM?
Guides I see in gamefaqs are based on original KHII, not FM.
If you just beat the game, it can get kind of overbearing thinking about all the extra content. This is what I did. It may not be the most effective way to get the platinum, but it worked for me. And if you're not interested in the plat, just skip over the things that you don't want to do.
1. Beat all Absent Silhouettes/Beat Sephiroth
2. Max out your forms and summons.
3. Collect every puzzle piece and chest that you can.
4. Clear the Cave of Remembrance
5. Synthesize 8 Lucky Rings.
6. Synthesize Everything.
7. Do the Data Fights/Lingering Will/Get to level 99/Mushroom XIII
8. Minigames, Missions, and Coliseum stuff.
I did all the gummi ship stuff as I got to them in the main quest. You should have some decent to good gummis since you beat the game, so you can do the gummi ship stuff whenever. I'd just knock it out on its own, and if not, take a break from it to do something else. The cool thing about the extra content is that you can stop when you want.