Visit Shrine of the Thunderblade in North Stonegard Pass. The shrine entrance is west / southwest of the save point here. Enter and press A on the shrine and achievement pop.
North Stonegard Pass is on the way to Cyrus’s Chapter 2 story. From Cobbleston, head northeast to North Cobbleston Gap, southeast to North Stonegard Pass. Check the signposts if needed in each area.
I wouldn’t rush getting this or the other secondary job achievements as the areas have much harder battles compared to the starting areas for chapter 1. Be at or just a few levels below the “danger level” shown on screen when you enter areas.