To get this achievement you must complete Mokujin level, which must first be unlocked.
In order to unlock the level you must first unlock hard mode by progressing through the story, and replay Leo's level (16th Archaeological Expedition's Excavation Site) under hard conditions. When completed you will unlock Mokujin Level.
Simply complete the level to unlock the achievement. Note that you don't have to beat Mokujin level in hard mode, so change difficulty to medium If you're struggling