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