Many Faces is a world boss in Yaesha. You should get this world boss if you get the Red Throne starting location. When running into the fight, the boss name will be shown as the Corrupter. The fight consists of a few mechanics. When running into the fight the boss will have around 67% HP and have a minion with large health bar attacking you. Aiming for critical damage and making sure to destroy its' hand attacks, you can kill him and cause the eye-like monster to come revive him. Shooting him from behind and under him will do a ton of damage and you will likely send him to < 40% hp. At this point you will likely hit his phase 2 threshold. The fight remains the same, except the eye-like monster will shoot a beam which will damage you if you're in front of it (full distance). If you get hit with the beam + the minion combo you will likely die. To avoid the beam attack you can vault back and forth between the two bridge-like land structures. Upon killing the eye-like enemy, the achievement will pop. Please note, you do not need to kill the minion again, you can just keep firing on the eye-like enemy.
Video of my full kill below: