Here is the full list of all 12 Flashback achievements worth 400 gamerscore. It takes around 15-20 hours to unlock all of the achievements on Xbox 360.
You have reached 100% in World Exploration
You have obtained the maximum level for every upgrade
Conrad has reached level 30
You have killed 200 enemies with explosives
You have killed 500 enemies with the Heavy Shot
You have killed 50 enemies with a melee attack
You have killed 50 enemies with an Execution
You have completed the game in Easy mode
You have completed the game in Normal mode
You have completed the game in Hard mode
You have completed all VR challenges
You have killed an enemy by finishing him