Also, what is the best way to spawn/respawn the special/rare heartless in each world? Should I leave the world and re-enter? Kill the regular enemies (if they spawn) and zone 2 screens away and come back? Thanks for any help. :3
Encounter Up doesn't seem to do anything for rare Heartless. For most of them, simply moving two screens away and then returning will occasionally cause them to spawn, but some are notoriously difficult to spawn with that method after the first time so running to a save point, leaving the world and then returning puts the odds more in your favor (same for soft resetting, I suppose). For me, that was Pink Agricacus in Deep Jungle. No way to know if that was intentional, it's just that in my first Final Mix run even after two-screen-awaying more than 20 times it never appeared again. Not two tries after leaving the world and return, there it (or they) were again.