Zone: Stepping Stones
This achievement is done in the city area (stealth trial) and should be done after earning the Difficulty courts achievement.
First, you need to acquire the Mace of the Guardian (weapon). You obtain it while progressing on the stealth trial (around the middle of it) when you need to interact with a male NPC and offer them 100 pieces of gold to give you the mace, then make the barrel explode and attract the guards to him.
If the achievement is attempted after obtaining the Difficulty courts achievement, just make sure you have the mace equipped, run to any guard to make them all hostile and slaughter them all! (Be careful with the commander, he's tough)
The achievement is awarded when all the enemies in the city are dead.