LEGO Marvels Avengers

Note: You must play as each character in a single session, meaning you can’t return to the main menu, play another game or turn your console off before spawning as all characters.

For this achievement, you first need to unlock every Iron Man variant (there’s 14 of them) then switch to and play as each of them in a single session. Some of the variants are automatically unlocked after completing a level while others you need to either solve puzzles or do side missions for other characters in the hubs. Upon spawning with the last set of armor, the achievement will unlock. 

• Iron Man (MK 1) - token located in the Hall of Armour in Tony Stark’s mansion in the Malibu Hub (needs Telekinetic Powers ability) for 10,000 studs
• Iron Man (MK 5) - token located in the garage in Tony Stark’s mansion in the Malibu Hub (need Electrical Panel, Scan Vision and Repair abilities) for 50,000 studs
• Iron Man (MK 6) - automatically unlocked after completing the level ‘Shakespeare in the Park’
• Iron Man (MK 7) - automatically unlocked after completing the level ‘Avengers Assemble’
• Iron Man (MK 16) - token is located in the level ‘No Strings On Me’ in Free Play (Part C) for 16,000 studs
• Iron Man (MK 17 - Heartbreaker) - token is located in a secret area of Tony Stark’s mansion in the Malibu Hub (need Pass through LEGO Grates ability) for 17,000 studs
• Iron Man (MK 25) - token is unlocked after completing the S.H.I.E.L.D. Scientist’s first mission (Helicarrier Laboratory), ‘Oh Cho They Di’nt!’ for 25,000 studs
• Iron Man (MK 33 - Silver Centurion) - token is unlocked after completing Agent Coulson’s second mission (Upper East Side at Central Park entrance), ‘Lola Larks’ for 33,000 studs
• Iron Man (MK 40) - token is unlocked after completing Agent Coulson’s first mission (West Harlem in Manhattan), ‘Duelin’ for Fuel’ for 40,000 studs
• Iron Man (MK 42) - automatically unlocked after completing the level ‘Ready A.I.M. Fire’
• Iron Man (MK 43) - automatically unlocked after completing the level ‘Struck Off the List’
• Iron Man (MK 45) - automatically unlocked after completing the level ‘Ultron Undone’
• Superior Iron Man - token is located in the parking lot next to the Avengers Facility (northern Manhattan) (need Cosmic Powers ability) for 50,000 studs
• Hulkbuster - automatically unlocked after completing the level ‘Anger Management’ (this one is only selectable when changing from Tony Stark; you can't select it from the grid)

Thanks to VGFAQ for the Iron Man variants video: