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When progressing through Act 3 (Babylon), you eventually reach the Great Wall area. When walking up the Wall, an NPC will prompt you to help them clear the Wall of a group of fearsome Peng monsters.
As you walk the linear Wall's path, you will defeat a number of Peng mob packs. Reaching the middle of it, you will face a Peng boss monster.
Defeating the boss monster, you will be granted the 'Be a man!' achievement.
This achievement is practically unmissable, unless you speed through the area, ignoring the boss monster.