This is unlocked after you defeat the final boss, Heng Long, and watch the final cut scene. Long has various kinds of attacks. One is a fire arc, which you can simply jump over. You'll know he's about to do this if he swings his sword in a figure eight like formation. His linear fire expulsions must be dodged by means of running or rolling by tilting
in various directions. Periodically he'll rise into the air and throw homing fire balls at you. You can dodge them with rolls or move away from their paths while repeatedly jumping, which also works very well. If he rises into the air this may also mean he's going to smash the ground. This attack raises several spikes around his location. While the spikes are still up, he may begin a new attack. Another attack he uses is a simple combo of two slashes. Later on in the fight, he'll summon meteors. Before impacts, yellow circles form on the ground. Look out for these, running around and rolling to avoid them. These may also damage Long himself. I would recommend using Patan's Tusk to hit him since it dishes out reasonably large chunks of damage with each swing. During the second phase of the fight you won't have any weapons or abilities, so you should be much more careful. Around the middle of this part of the fight Briggs will rip out Long's heart in a QTE. This doesn't weaken him much, but it does take away a lot of health. Once his health bar goes down to zero after some more fighting, the final cutscene will begin. Only by decapitating Long and piercing his head with the business end of Kukulkan's Fangs is Briggs able to finally defeat him. This gives Briggs control of Long's sorcerer powers. After the cut scene is over, you'll unlock this achievement.