Here is the full list of all 52 Astellia achievements.
Equip [Legendary] Armor for the first time
Equip a [Legendary] Weapon for the first time
Complete all Main Quests of Athlon: Traces of the Legend, The Way to Lagash Underground Sanctum, Rashan's Mistake
Clear Hashmal Laboratory Scenario Dungeon
Discover Almantin Village, Humbaul Fortress and Hashmal Laboratory for the first time
Clear Lagash Underground Sanctum Scenario Dungeon
Reach Rank C in Colosseum for the first time
Equip [Heroic] Armor for the first time
Reach Character Level 50
Complete the Lentuth sub-quests: Fighter of the Wind, Astel of Aquarius, and Black Goat of Sansara
Complete all Main Quests of Lentuth: Dawn Howl Subjugation, Witch of the Abandoned House, Dogavi Night Market, Maglev Union
Clear Sansara Fortress Scenario Dungeon
Discover Ivera Fortress, Anatemo Village and Morara Plains Fortress for the first time
Clear Dogavi Black Market Scenario Dungeon
Equip a [Heroic] Weapon for the first time
Clear Archduke Caleonid's Mansion Scenario Dungeon
Reach Character Level 40
Reach Rank A in Colosseum for the first time
Clear Fortenora Underground Prison Scenario Dungeon
Reach Rank S in Colosseum for the first time
Clear Temple of Blessings Scenario Dungeon
Discover Frel Village, Twilight Village and Temple of Blessings for the first time
Complete all Main Quests of Randeilo: Stellar Heritage, Attacking Macarzan Basin, Reclaiming the Temple of Blessings
Complete all Hidden Chest Achievements in Randeilo
Clear Macarzan Basin Scenario Dungeon
Reach Character Level 30
Complete the Randeilo sub-quests: A Merchant's Secret, Butterfly Fairies in Danger, and Foxy New Comrade
Clear Hall of Equality Scenario Dungeon
Clear various achievements and collect 1,000 Achievement Points
Own 20 or more Astels
Clear Arau Cave Scenario Dungeon
Complete all Main Quests of Ragferant: Rutlass Bandits, Orcs of Crimson Rock Valley, Final Chance
Complete the Ragferant sub-quests: Taking Care of Kiyo Kidnappers, Eline Falls Purification, and How to Stop the Epidemic
Complete all Hidden Chest Achievements of Ragferant
Equip [Rare] Armor for the first time
Clear a total of 10 Dungeons
Own 10 or more Astels
Discover Meiville, Full Moon Market and Rumbling Axe Outpost for the first time
Clear Crimson Rock Valley Scenario Dungeon
Equip a [Rare] Weapon for the first time
Clear various achievements and collect 100 Achievement Points
Clear Rutlass Den Scenario Dungeon
Complete all Hidden Chest Achievements of Lentuth
Complete the Athlon sub-quests: Broken Trust, Assassin in the Desert, and Faulty Experiment
Complete all Hidden Chest Achievements of Athlon
Complete all Hidden Chest Achievements of Ragferant, Randeilo, and Lentuth
Own 35 or more Astels
Clear a total of 100 Dungeons
Win an Avalon
Win a total of 7 Avalons
Defeat Avalon Bosses Blue Blood Warrior Alror, White Opalu Wolf, Cacodemon, Sunok the Great Witch, Protector Roa and Rokas, Spawn of Hellfyre
Clear various achievements and collect 3,000 Achievement Points