How do i get the achievement on halo 3 : odst , called "Good samaritan"?

[Nick]

i have been playing halo 3: ODST for a while, but i can't achieve the this achievement.
The information given is as following:

Good samaritan

Killing things that are new and differen is bad, alone or with another ODST.



my gamertag : nickth271
email : [email protected]

 

[Kolped™]

start playing Prepare to drop as the Rookie and finish the entire game without killing a single engineer