One from Persona 4:
Matsuo's shadow. Apparently loads of people found this really hard, but I just spammed my most powerful physical attacks from Kanji, Chie and my character then had Yukiko use Mediarama every turn. It's easy when you've got three characters with 400+ health and one with 400+ SP that has the skill that makes health spells 50% better. I did kind of grind a lot in that dungeon before going into the boss room, so I was about 10 levels above the recommended level. I was also lucky and ran into about ten golden hands in that dungeon, each giving me 10,000+ free EXP