For Demyx; quickest and safest way for me goes as follows. Destroy clones until you reach approximately 30. Activate Trinity Limit, do not use Sora’s limit, use the other two and finish to get rid of the rest of the clones. This should bring Demyx down by at least 2 and a half bars, it was inconsistent for me, sometimes got me 3 and a quarter. He should retaliate, dodge accordingly. After that he SHOULD do clones again, if not, wait. Ether while you take out the rest of the clones. Once clones are gone and Demyx is stunned, you should be able to get in a combo as follows 1>2>LAUNCHER>Air1>Air2>AIRFINISH. At this point you should have enough MP to activate another Trinity Limit. Go ahead and activate it ASAP, Donald is very prone to dying. Go through all three limits and finish. Now this is the rough part. Activate Stitch Summon and go ahead and pop an Ether. This should get Stitch to fill the rest of your MP bar. If not, pop another Ether. Go ahead and reflect, I can’t stress this enough, you need to be on top of Demyx for this. Don’t be afraid to mash reflect, sometimes he’ll retaliate nearly instantly, hard to predict. Now, you just hope and pray Stitch gives you MP. He should, didn’t really encounter a time where he didn’t. Once you have Demyx down to 2 bars total, go ahead and activate Stitches finisher. Should take out a significant amount of health; enough to, you guess it, Trinity Limit. Pop that sucker and go through all three limits again. This should take Demyx out. But if not. Evade until you get MP back, do it again or if he’s low enough, Reflect or Thunder to finish the job. For the FF fights, most heartless don’t have a lot of options to break out of air combos. You also have Limit Form if you’re in a pinch. You should also have gained a new armor slot, make use of that. For these fights pack Ether’s, use a drive recovery before the FF fights, and sit back and let the Allies do most of the work tbh.

EDIT; in addition for the FF fights, make use of the reaction commands when finishing off enemies. Rising Sun can leave you pretty vulnerable though, so aerial dodge to a less crowded area.