To get this achievement you need to re-roll your Campaign or Adventure mode on Reisum until you get to the side dungeon named
The Frieran Sea. Once you get there you will see a wrecked ship, however off to the left will be a small path with boxes blocking the way. Break them and crawl under to reveal a small platform you can climb, once up you will see a prompt to look into a hole in the side of the ship revealing the
Warlord's Armor Set.Now you can backtrack and head inside the ship. Once inside you'll see stairs leading down and a path straight ahead, take the path where you'll meet
Sebum, A fat two-headed rat. Talking to him will reveal that he wants you to kill a Packmaster, which are the yeti type elite monsters that randomly spawn in Reisum. You can now leave The Frieran Sea and explore around until you find a packmaster and it drops the
Master's Tusk. Keep in mind that the drop chance is NOT 100% and it may take you a few tries. Once you have it however go back to Sebum and SLOWLY talk to him and make sure you choose the options talking about the Armor below the ship.
You will then have TWO options:
1. I found the Packmaster. Here's its tusk. [GIVE TUSK]
2. I have the tusk, what about that armor below deck?
Choose the second option and then choose "This Tusk, for that Armor" Sebum will cave in and accept your offer and then the door down stairs will unlock and the armor will be yours to claim, the achievement should unlock as soon as you finish speaking with him.