This is an optional boss midway through the most northern region of the map called "Elphael, Brace of the Haligtree". This is an endgame area you reach after finishing most of the story. For reference I was 60 hours into the game when I reached this zone and at a player level at around 120+ (without grinding).
The way to this location is very convoluded.
To access this area you have to first make it to the hidden path to the haligtree, left of the story related Mountaintops of the Giants. In order to get there you need two halves of the "Haligtree Secret Medaillon". One you can find near the "Village of the Albinaurics" grace site in Liurnia. Its left of the "Scenic Isle" checkpoint on the big lake underneath some mountains. There you will find a NPC hidden in an pot you have to hit in order to unveil him (up the hill). He will give you one half after talking to him.
The other half can be found at the end of the "Castle Sol" which can be found in the northern region of the "Mountaintops of the Giants".
When you have the complete medaillion head back to the "Grand Lift of Rold". You will have the ability to select a new destination for the elevator which will now get you to the new area.
After reaching the "Hidden Path to the Haligtree" head north until you reach a seemingly empty city called "Ordina, Liturgical Town". There you will have to solve a puzzle by activating 4 flames on the rooftops after entering a sort of teleporter.
After completing the puzzle you will be able to teleport to the location needed for the achievement.