First tip: Dont grind all 7 it will take too much time to max all of them choose 3 optimal for your team and the ones you mastered best. - Equip accesories increasing exp gain on all of them. - 65 to 75 maybe 80 if you are extreme you can grind on final level of last boss dungeon(radio tower) it will take you some time it is hard to die there and take less time than fighting dragons. - Also grind on map dragons in the extra dungeon the respawn each time you leave dungeon are esier to kill than random encounter ones and give good exp sp also may drop very good items (although they have low drop rate).
Or don't grind at all for level. This game doesn't require you to grind at all to beat any bosses. You only need to know how to use your skills efficiently and you'll do fine, Also, If you want to have the strongest weapon in game you're require to fight against the Dragon (Purple Icon) in the secret dungeon.
So, either you go to Tokyo Tower and grind all the day for levels that only add a few minor digits or a set-up skills + equip that can defeat bosses?