Finally got my four Mega Flares so I just tested it. Mega Flare gives you a guaranteed 30 LP and AP roughly every 25ish seconds on top of EXP for Riku and drops from the enemies which includes about 100ish Munny but both Mega Flares need to be shot at the center groups both times and you need Treasure Magnet 5. There will also be a delay of about 5 seconds if you don't have at least one Magic Haste on your DEs. Balloon takes roughly 5ish seconds and gives you 12-41 AP and 10-30ish EXP but costs 64 Munny every attempt and you have to restock it. Candy Goggles give the same but instead of EXP you get LP and it's about 10 seconds to do it. I think what you need to determine is what's best for you, if you want to fill out the boards or just affinity. The best method is to buy 99 Balloons, use them, then spend the rest of your time doing the Mega Flare method to earn your money back and repeat. Or you could do Candy Goggles which gives you LP and lets you max out the board before switching over to Balloons and running the Mega Flare method when you need Munny.
For PS4 items are definitely the best way to go around maxing both boards and affinity, the best way to sustain it would be to get Munnystorm on Symphony of Sorcery with Riku and just farm for Munny there then go to a world that also has the Bargain Storm I think it's called to get everything at a discount but that takes a lot of luck to land on the same drop. On 3DS Mega Flare is the way to go due to load times and how long it takes to quit out of either game into the bonding menu and going back to the toy. Munny method via Symphony of Sorcery also works on 3DS but again load times for items are way too long to justify it.