Cross of the Dutchman

During Chapter 4, and after finding Peter, you'll be tasked with buying a health upgrade at the store in Kingswerd (the map icon is a potion bottle). If you've looted enough gold (you'll need 350 in total), buy both the health upgrades (you don't need to purchase the Stamina upgrades).

Once you've bought both health upgrades, return to Peter where you'll then be tasked with defeating the Saxon forces in the south of the map. Upon defeating all enemies, you'll be rewarded with the achievement.