You will need to get every character to level 200. Save this for a second or third playthrough, as you can buy a perk for 3000 from the Grade Shop that will give you 10x the experience from all battles. You start each new playthrough at level 5 though, so don't grind too much on your first play thinking that will help. Make sure you unlock the EX Dungeon as the normal experience from bosses in there is 30,000, making it 300,000 with the 10x perk. Giganto monsters are also a good source of experience.