From Halopedia, the Halo wiki
Good Samaritan is an Achievement in Halo 3: ODST. It is awarded for not killing any Engineers during a full campaign run through, in single player or Co-op. It is worth 20 Gamerscore. This achievement will unlock the full ODST Ballistic Battle Armor for the player's Xbox LIVE avatar on Halo Waypoint.
It is represented by an ODST helmet looking upwards shown in a white light.
- This achievement only requires the player to not kill any Engineers while they are playing as the Rookie in the Campaign Level "Mombasa Streets." The achievement is awarded at the beginning of the level "Data Hive".
- It is recommended that players attempt to unlock this achievement before unlocking Naughty Naughty achievement for killing 10 Engineers. This is because if the player accidentally kills an Engineer, an on-screen alert will notify them of progress made toward Naughty Naughty. This will prevent the player from going through the entire game, believing they are on track to receive the achievement, only to be disappointed at the start of Data Hive.
- An easy way to get this achievement is to set difficulty to Easy and use a Mongoose (if unlocked from the collection of Sadie's Story) to skip fights and just go through the various flashback levels.