Here is the full list of all 50 Injustice: Gods Among Us achievements worth 1,000 gamerscore.
Send an opponent through all three Metropolis transitions in one fight
Win all story mode minigames
Unlock all costumes in the Archives
Complete Classic Battle with all characters
Perform every character's supermove
Complete Tutorial
Complete 1 S.T.A.R. Lab Mission
Get 3 stars on 1 S.T.A.R. Lab Mission
Cause maximum damage in all Arenas (does not include Practice mode)
Equip a new Background Image
Equip a new Icon
Complete Classic Battle with any character
Unlock a costume in the Archives
Complete 100% of Story Mode
Complete 50% of Story Mode
Knock an opponent through a transition
Win a Clash sequence with any character
Reach Level 10
Win a match with the super move of any character
Equip a new Character Portrait
View Your Hero Card
Complete Battle Mode
Perform 8 throws and win in a multiplayer match
Use every interactable and win in a multiplayer match
Win a multiplayer match with a timeout
Beat All S.T.A.R. Lab Missions (Excluding DLC)
Reach Level 100
Win a single online match
Dethrone the King in an online match
Vote Correctly in a KOTH match
Defeat a Survivor
Win 100 complete Online Matches
Play 200 complete Online Matches
Win 1 complete Ranked match
Win 10 complete Ranked matches
Enter Online Practice with someone on your friends list
Enter Practice Mode
Get 100 Stars in S.T.A.R. Lab Mode
Complete All S.T.A.R. Lab Missions with 3 Stars
Perform a 10 hit combo with every character
Knock opponent through all transitions across all levels
Perform every special move of every character
Perform every Meter Burn special move of every character
Win a match with Green Arrow using only arrows
Win with Batman using every special move and his Supermove
Perform every level interaction once
Defeat every Villain With Batman
Make a comeback when at low health (10% or less)
Win a match as Deathstroke without missing a shot (minimum 12 shots)
Unlock everything in Archives