Here is the full list of all 18 Guardian achievements.
Defeat the first boss.
Defeat the second boss.
Defeat the third boss.
Defeat the fourth boss.
Defeat the fifth boss.
Defeat the sixth boss.
Defeat the seventh boss.
Beat the game.
Gain all four spells.
Upgrade each of your spells.
Acquire your first skill.
Acquire all skills.
Gather all keys.
Gather all health fragments.
Gather all augment orbs.
Obtain 100% total completion.
Slay 500 enemies.
Use your downward smash on 20 enemies.