Zone: Stepping Stones
This achievement is done in the city area (stealth trial) and should be done before attempting the Berserker thief 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.
After obtaining the mace, equip it, complete the main story quest, teleport back to the beginning and enter the city island again. (Don't unequip the mace until you get the achievement)
To earn this achievement, you need to cross the city again without being caught by the guards (*IMPORTANT*: Save very often) You will know when you are caught because the guards will yell, turn hostile and run to attack you. At this point you have to reload to a previous save because the achievement is lost.