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:
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.