Harp Zu Hestui:
Listen closely then. Go to the Blazing Swamp, kill local monsters and collect 1000 pieces of Hot Energy from their corpses. Or you can do the same at the War-Torn Plains, if you want.
You will also need to reach Lv. 100.
When you're done, go to Dual Class Master Joachim. Use the Teleportation cubic to return to him.