Here is the full list of all 21 Yu-Gi-Oh! Legacy of the Duelist achievements.
Win all Duelist Challenges
Perform a Pendulum Summon
Win an unranked multiplayer match (duel)
Win a ranked multiplayer match (duel)
Deal 50k in Reflective Damage
Unlock all avatars
Chain 100 times
Win a duel by having all Exodia cards in hand
Defeat ZEXAL Campaign
Defeat 5Ds Campaign
Defeat GX Campaign
Deal 100k in Effect Damage
Create 5 decks
Win a Battle Pack match (duel)
Defeat Classic Campaign
Deal 200k in Direct Damage
Perform a Fusion Summon
Perform a Tribute Summon
Perform a Synchro Summon
Perform a Xyz Summon
Create your first deck