Here is the full list of all 50 Marvel's Avengers achievements. It takes around 100-120 hours to unlock all of the achievements on Windows.
Defeat 10 or more enemies with a single Heroic ability activation
Collect a full comic set
Reach the VIP balcony
Reach Power Level 150 with any Hero
Earn a total of 250 Hero Levels across the entire Hero Roster
Reach rank 25 in any faction
Complete 10 assignments
Reach Hero Level 5 with five different Heroes
Reach Hero Level 50 and purchase all skills for any Hero
Complete 15 Villain Sectors
Complete 100 assignments
Open 50 Caches strongboxes
Complete five Hive missions
Complete the "Iconic Avengers" mission chain
Complete the "A Global Offensive" mission chain
Complete 50 Hive missions
Reach a combo of 20 hits or higher
Earn a 100% rating on a mission without any team member being downed
Earn a five star rating on an objective without any team member taking damage
Complete 30 War Zones at Challenge III or higher rating
Earn a 100% rating for any War Zone at Challenge IV rating
Complete any mission in under three minutes
Defeat any 20 different enemy types
Complete HARM Challenges I to V
Collect 50 intelligence files
Collect 500 gear items
Collect three Artifacts
Rescue 100 AIM prisoners
Equip all Legendary or better gear with any hero
Defeat 50 enemies with Assault Heroic or Ultimate Heroic abilities
Fully upgrade a major artifact
Fully upgrade any gear item
Purchase an item from two different faction vendors
Break into 30 Depots
Open 100 strongboxes
Complete an Elite Heroic Hive
Escape the clutches of AIM in New Jersey
Discover the location of the Chimera
Recruit Bruce Banner
Recover the SHIELD Protocols
Recruit Tony Stark
Destroy the AIM arc reactor
Locate the Ant Hill
Recruit Black Widow
Rescue Inhumans from the research facility
Stop the Helicarrier from crashing into New York City
Reach the AIM space station
Rescue Captain America
Reunite the Avengers
Defeat MODOK and the Kree