After completing Tressa’s chapter 1, head to the marked town of Quarrycrest to start chapter 2. After some talks and merchant activities, keep following the green markers and defeat Omar and the two Footman. After some scenes in town the achievement will pop.
Omar is weak to spear, knife, staff, ice and light. The two Footman are weak to sword, spear, lightening and wind. Your Warrior skill “thousand spears” is great to break Omar’s shields as well as second level ice spells from your Scholar class. After breaking Omar, he’ll recover with two additional shields, up to a total of nine. I was able to keep him broken quite a bit of the time - he hits hard so focus on breaking him. Some SP potions are good to have to keep your Warrior and Scholar classes ready, and Cleric using charged healing spells for the group as needed.