Here is the full list of all 26 NERO achievements.
Complete mission : Redemption
Kill Total of 500 enemies
Complete mission : 2nd Life
Complete mission : On the Trail
Complete mission : Back to School
Complete mission : Trapped
Complete mission : Hidden Cargo
Kill 50 enemies with explosive barrels/generators
Kill 100 enemies with pistol only
Complete mission : The Tower
Kill 100 enemies with rifle only
Complete mission: The Bomber
Kill 100 enemies with only head shots
Complete mission : Ghost of the Past
Kill 50 enemies with sniper only
Kill 50 enemies with Grenades
Kill 50 enemies with mines
Kill 5 enemies with one Grenade
Complete a mission without taking damage
Complete a mission without taking any health packs
Kill every enemy in a mission
Find all the intel documents in the game
Complete a mission without being detected
Kill 50 enemies with take downs or melee attacks
Complete every mission in "Brutal mode"
Get 100% accuracy in sniper training