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