I am not quite sure how I got this achievement as I had previously completed ODST and had killed an engineer in the city so I did not get the achievement.
But tonight I joined three friends who were playing through the game on Legendary difficulty (doubt it matters). I joined them at the start of Kikowani Station. We played through Kikowani Station and Mombasa Streets(F) and when we started Data Hive the Good Samaritan achievement unlocked for myself and one of my friends but not the other two friends.
I have no idea why it unlocked as I did not start the co-op campaign from the beginning but thought I would give it a mention here as it could be a glitch or maybe a workaround to get it faster by playing those two levels.
For proof you can see my campaign stats at http://www.bungie.net/stats/halo3/gamehistoryodst.aspx?playe...
- the three games on 20th October.
Hope it helps in some way.