Here is the full list of all 30 The Elephant Collection achievements.
Beat your first Rave in Elephant Rave
Unlock Rapidfire mode
Unlock Reverse Mode in Obey! The Game
Clear 60 Stages in one game in Obey! The Game
Lose your memories in Elephant Rave
150x Final Rave in Elephant Rave
Achieve 25 percent of Achievements in Achievement Unlocked
Achieve 50 percent of Achievements in Achievement Unlocked
Achieve 100 percent of Achievements in Achievement Unlocked
Clear Stage 5 in This is the Only Level
Clear Stage 15 in This is the Only Level
Complete This is the Only Level
Collect 100% Achievements in Achievement Unlocked 2
Hit 25% complete before leaving the first floor in Achievement Unlocked 2
Play Call Me Maybe On the Piano (CEFECCEDDC) in Achievement Unlocked 2
Clear Stage 30 in This is the Only Level 2
Unlock the Velociraptor skin in This is the Only Level 2
Find Stage 31 of This Is The Only Level 2
Defeat Wooly in Elephant Quest
Build an army of 10 Miniphants in Elephant Quest
Reach Elephant Level 12 in Elephant Quest
Collect 100% Achievements in Achievement Unlocked 3
Buy exactly 50 pellets at once in Achievement Unlocked 3
Reach combo 20 hamsters while wearing the pirate hat in Achievement Unlocked 3
Complete This is the Only Level 3
Spell the only English word you can with all seven letters in Stage 14 [Curable] in This is the Only Level 3
Input Pi on Stage 25 extremely quick [in 8 seconds or less] in This is the Only Level 3
Complete Run Elephant Run
Complete Run Elephant Run in Hard Mode
Complete Run Elephant Run in Insane Mode