How to complete the Opening the Arena side quest in Horizon Forbidden West

gamepur.com:

Opening the Arena is a side quest in Horizon Forbidden West that you can pick up after completing The Kulrut main quest. Two women are standing in the center of The Grove, and when Aloy approaches, they explain that they need some machine parts to get the Arena back up and running again. This guide explains how to complete the quest so that you can start ripping apart some machines.

To complete this quest, you need to bring back two Bristleback Tusks and a third part from a Rollerback. You don’t need to do anything special for the Rollerback part other than beat it, which is a challenge in itself. Rollerbacks are weak to acid damage, so try to use that if you have the right weapon to do so. Shooting weak spots deals increased damage, and any Spike Thrower you have will shear off parts.

The Bristleback Tusks can only be obtained by tearing them off the Bristlebacks while they’re alive. This means that you must use concentration to focus your aim and hit the tusks a few times before they fall off. If you have a weapon with a high tear damage rating or Tearblast arrows, those will be best for removing parts.

When you’ve collected the parts, return to the quest giver to complete it. This will unlock the Arena for you to train against all manner of machines. It also unlocks a new store where you can spend Hunting Grounds Medals and Arena Medals on some of the most powerful gear in the game.