KINGDOM HEARTS HD 2.8 FINAL CHAPTER PROLOGUE (EP)

This can be completed over multiple playthroughs

You cannot play on Critical difficulty until you have cleared the game for the first time, so I wouldn’t suggest grinding for this one. Clear the game once on any difficulty, and on your second Critical playthrough (which you’ll need to do in order to unlock the Ambitious achievement) this will unlock naturally.

Otherwise in my first completion I had destroyed over 400 heartless, so it wouldn’t take long to grind this out if you wanted to. I would suggest using the middle mirror in the World Within. Run straight up through the three mirrors then turn around to get the last mirror. Here you’ll find 30 odd heartless and they respawn immediately meaning you can also use this location to grind exp.