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.
Guide to all of BOB's location in Winter Contingency:Update:
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.Update 2 (January 8th, 2020):
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.