It involves a good deal of grinding. Find every red journal on the way to the end. When you beat the minotaur and leave the school, theres a path to the right that leads to the home of the person that wrote the journals. Read the journals in there then go through the fireplace. You'll find a door you can't open. Create every single magic orb except for the god ones. Then go back to the school courtyard teleport(where yuuki is sleeping). Go to the northwest exit and then through the hedge maze. The fairy at the end will have 2 crafting options. Make both of them then head back to the door in the fireplace. After you open it, equip them both(they are very strong) and you'll have a boss fight in there. Once you win, you'll get the item you need for the true ending. You can equip it in your accessory slot to basically one shot the normal final boss.