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