Well...Maybe One or Two Achievement in Halo: The Master Chief Collection

As one of the only new “enemy” types in Halo 3: ODST, Engineers will be found frequently during the eight missions of the Campaign and around the streets of New Mombasa. Though players will most likely not even kill more than 10 during a natural playthrough of the Campaign, there’s a perfect “grinding” spot where this achievement can be earned quickly. 

During Kikowani Station, proceed through the mission as normal; grab a Banshee, wait for your allies to board a Phantom, open the first gate, head through the service tunnel, and emerge into the second outdoor area. In this area, begin to pilot the Banshee north towards the next objective and you should grab a checkpoint mid-flight. What’s more, directly ahead of you are four Engineers hovering around a Covenant beacon. As soon as you spot them, open fire with the Banshee’s turrets and fuel rod cannon, though take note that even on Easy these aliens take quite a few hits to kill. Once all four Engineers are dead, revert your checkpoint and continue to grind out the kills in this location until the achievement unlocks.