Here is the full list of all 15 Avoid The Monsters achievements.
Beaten all small levels on casual difficulty
Beaten all large levels on normal difficulty
Beaten all normal levels on normal difficulty
Beaten all small levels on normal difficulty
Beaten all normal levels on casual difficulty
Beaten all large levels on casual difficulty
Beaten all huge levels on casual difficulty
Beaten all mega levels on casual difficulty
Beaten all huge levels on normal difficulty
Beaten all mega levels on normal difficulty
Beaten all small levels on hardcore difficulty
Beaten all normal levels on hardcore difficulty
Beaten all large levels on hardcore difficulty
Beaten all huge levels on hardcore difficulty
Beaten all mega levels on hardcore difficulty