Return to the castle and the throne room for a cutscene. Then, go down and to the left to the prince's bedchamber. Head to the gates of Gallopolis and leave. Then, go West and North, through the checkpoint. Go to the northeastern campsite on the map (
). Follow the path into the cave. You'll fight the Slayer of the Sands after you see all the horses lined up. As always, this fight is easier if you are Pepped up. The Slayer has a ton of health, so prepare for a long battle. I was around level 14 for this fight and found it really easy.
Return to Gallopolis for a long cutscene. Now it's time to leave Gallopolis again. Head West to Laguna di Gondolia. Continue west and then north into Gondolia. In Gondolia, go left, up the stairs then, turn right, walk a bit, and on your left, take the next set of stairs down. Here, head south into a cutscene. The house you need to go to is on the bottom second to last along the east side of the city. Open the door for a cutscene. Leave Gondolia the same way you entered.
Go West into the Grotta Della Fonte. Here you'll need to fight a tough boss: two Coralossuses. My party was level 15 and I found this fight hard. They have high armor, so hit them with magic attacks. Once they are defeated, get the holy holy water and return to Gondolia. When the curscene ends, head straight forward into the next cutscene. You are forced to lose this fight. After another cutscene, you'll have a stealth section, but I just ran and fought my way to the center of the bridge. Now, jump off the bridge, and get on a boat. Take the boat to the dock on the left.
This leads you into a fight with the knight. He's quite dangerous. He can increase his defense or prevent you from casting spells. After defeating him, there'll be a cutscene, and then the achievement will unlock.