halo 3 help needed please !?

[gamer 4 lyfe]

i want the odst armour for my xbox avatar as i have the helmet and i herd you get the armour for doing the good samaritan achievment which is to go through all the campaign without killing any of a certain enemy

what does this enemy look like? i know what its called but cant reember lol but ive never herd of it
also must you play through it all or can you skip prepare to drop and the roaming the city parts
also must this be done in legendary? or can i do it in easy to get through faster

thanks guys

 

[Miguel]

i think its the Engineer, they float around and they look like turtles if you think about it