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