If you've already progressed through Chapter 4, there's a chance you might already have the Armorcharm in your Item Box!
The message is incredibly easy to miss and it doesn't appear in the usual Quest Rewards area.

Otherwise, you'll need to progress until the Chapter 4-2 Side Quest, "Secure Wounded Hollow".
The quest is given by Cobb at Suja (a guard that typically hangs out near your tent area).

To complete the quest, you'll need to simultaneously hunt 2 Hirabami - see our Hirabami guide here if you need a reminder on how to beat it.
As a quick refresher, it's weak to the
Fire and
Thunder elements along with
Poison and
Sleep status effects.
The main thing here though is to take advantage of the levers that wall the arena in half.
If you can isolate and focus down a single one, it'll make the fight much more feasible, especially if you're hunting solo.