This requires you to kill a golden-armored Ranger elite that spawns randomly within each level.
I recommend the second mission Winter Contingency, and splatter him with your truck. This can be done on easy difficulty.NOTE:
Only 1 BOB spawns every mission, and their location is random depending on its spawn points.
BOB in ONI Sword Base:
*** Spoiler - click to reveal ***Update:
Update 2 (January 8th, 2020):
I got a response regarding about the white elite dual-wielding plasma rifles in ONI Sword Base. This unique white elite is considered a BOB according to the game, and its armor permutation is random. Killing it will still grant you the achievement.
Since the the channel who posted the two guides I've linked is unfortunately deleted, here's an alternative guide for finding a BOB in Winter Contingency.