To earn this achievement, you need to defeat mezner, the final boss.
To do this, at the start of the battle, pick up any ammo needed from the right from the crates. Aim for the little light blue bulbs on the tentacle things sides, when you shoot the two off from one tentacle, equip your Glaive, throw it up to the lightning bar up top, then aim for the blue looking eye in the middle of the tentacles. Do this for this for all 3 tentacles.
After that, Mezner will be revealed from his chamber thing. Using your Glaive, you MUST!!!! make a power throw, if you do not do a power throw, you will die instantly. (If you don't know how to, hold "RB" until the crosshair turns yellow then immediately let go.)
Now Mezner will spawn two more tentacles. These are easier, just shoot the light blue orbs on their neck/body areas, (there is only one), then use the lightnining bar with the glaive, and throw the glaive at their eyes.
Mezner will pop out again, power throw him.
The final stage, mesner spawns about 5 tentacles, 3 in the front, 1 on each side of Hayden. Destroy the two on the sides first, as they have a tongue thing that can be very annoying. For these guys, you have to destroy the bulb on their neck/body, and on the back of their neck/body. (It's really easy to do), then use the lightning glaive on their eye part.
After taking out all 5, Mezner will be revealed again. Power throw him one last time to kill him and unlock the achievement.