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