Here is the full list of all 12 Frebbventure achievements.
Clear the tutorial
Gather 20 crystal shards in one world
Discover the first Secret History movie
Discover the second Secret History movie
Discover the third Secret History movie
Discover the fourth and final Secret History movie
Defeat Ulcethot!
Clear the "Earth's Strongest" boss rush mode
Clear "Earth's Strongest" without healing
Clear "Earth's Strongest" without taking damage!
Unlock all bonus characters in "Challenge Mode"
Defeat the true final boss!