Rainbow Pudding can be farmed with all jelly-type monsters, but Flan Princess has highest drop rate among them. Here is a strategy that I use (probably everyone else):

Flan Princess

• Cecil must equip Crystal Mail (the ONLY item that prevents Berserk).
• Equip Cecil with Stardust (must be first slot), Draw Attacks, Counter, Treasure Hunter augments.
• Swap him to back row.
• Now go to Lunar Subterrane - B5 (Southeast Room).
• Set to auto-battle, then heal Cecil when he is low in HP and repeat the Siren.
• You can choose to restart the game after 30 or 40 Siren (this way, you do not need to run back to buy Siren and Stardust).