The Cruel King and the Great Hero (EP)

To get this trophy you will have to level Yuu up to level 50.
This trophy will contain information on how to level up all characters to level 50, so the information here will also be used for the Silver Rocky, the Ice Fox, Silver Cybat the Trickster, and Silver Flora, the Flower Princess trophies.
When you reach Chapter 7 of Gold Great Hero, you can change party members by going to your home (the first Fountain teleport option, Before the Mountain Den) and talking to the character you want to choose.
Before you start leveling up your character to level 50, be sure to complete the Dream Eater 3 side quest (Chapter 7: Gold Great Hero) to unlock the Treasure Tome accessory, which increases experience gained in battle by 50%. Equip the Treasure Tome to your teammate rather than Yui, as she will always be in your party, so she will naturally reach level 50 while leveling up her teammates
To grind experience, I recommend teleporting to the Demon King Mt. fountain. Summit and in the cave between the fountain and the location with the final boss in Chapter 7, look for enemies, usually there are always 4 enemies at a time. To kill enemies in one go, you can use Yuya's Great Flame Slash skill. Then return to the fountain to restore energy, or use the healing items you can buy from the store next to the fountain to use in the next battle. Also, if you come across a Gloomy Raccoon among the enemies, then it is better to skip a couple of moves, since this enemy can use a skill to summon partners, thereby increasing the amount of experience received when winning the battle
It took me about 5 hours to level up all the characters to level 50