Here is the full list of all 12 Guardians Of Gaia: Guardians 8 achievements.
Defeat a Guardian
Defeat 16 Korac
Perform a Special move
Perform a Ulitmate move
Perform a weapon based Special move
Purchase a weapon
Perform an Air Weapon Throw
Complete the Challenge
Complete the Challenge with an B Rank
Complete the Challenge with an S Rank
All Achievements completed. Congratulations!
Complete the Challenge with an A Rank