I'm really fearing those gummi ship missings and synthesis trophies, but I'm going to do them asap to get them out of the way <_< I'd rather not dread having to do them to finish off the game completely, leaves a bad taste in my mouth haha
Some of the Gummi missions have really awkward and hard conditions to fulfill, like i.e. get a certain amount of points without collecting any item. In order to get points you need to destroy either enemies or obstacles, but both of them almost always release an item, so good luck going through a course without picking up any of those
and still destroy enough stuff to meet the point requirement.
My main gripe with the synthesis trophies is not that they're particulary difficult, but more on the tedious side because of so many materials from the rare Heartless being needed.
Most of those can however be done best after you have already finished most of the game, as you need the -ga spells to be most effective against most of the rare Heartless (Jet Ballon is a cakewalk with Aeroga) and Neo Shadows only appear in the final area of the End of the World before the last save point.
Chimaera (Halloween Town), Grand Ghost (Monstro), Pot Scorpion (Agrabah), Jet Ballon (Neverland), Neo Shadow (End of the Word), Stealth Soldier (Hollow Bastion) and Gigas Shadow (Wonderland) are rather easy to defeat and farm once you have Aeroga, however the Pink Agaricus (Atlantica) is a tad annoying because even with 13 MP I found Stopga lasting too short to get in more than 40 to 50 hits even with Aeroga and all combo plus-abilities equipped.
I contemplated ordering Donald and Goofy to attack as well, but I am not sure if their hits even count.
The Black Ballade (Deep Jungle) is also a tad annoying, mainly because I often can't follow it's movements closely, especially when Goofy stands at an unlucky spot and obstructs the view.
And then there is the Sniper Wild in Traverse, god, I hate that stupid monkey. The first three I can easily dispatch by stopping them and then whack away, but in the third wave there spawn three and my Stopga often only hits two of them, leaving the third to spot me before I can blast it apart.
I would also be interested to know if there is a sure-fire way to respawn the rare Heartless after you defeated one without having to leave the world, as this is what costs most of the time (besides playing Hide and seek with the Sniper Wilds).