ok, so i know how to kill the BOB banchee pretty easly.
this is what you have to do:
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start the level new alexandria and keep restarting the level until the objective club arera comes up first
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do the objective and go back in your falcon, keep shooting the normal banchees for about 2 minutes, BE SURE TO STAY CLOSE TO THE CLUB!!!
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after about 2 minutes a banchee that does not stops boosting comes raging by the club, just shoot that banchee (if you miss, restart to last checkpoint)
i have done this challenge and it works
just one other thing, i did this with one other person, and he killed the BOB but i still got the challenge completed, so if you have any other friends that need the challenge, go in 4 co-op and see if all 4 of you get it, otherwise you can also restart to last checkpoint and let the other persons kill the BOB
i do recommend this since it has to be on legendary, and you might die because of the normal banchees
how can you recognize the BOB?
the BOB never stops boosting and will probably not shoot at you but just rage by you, if any banchee does that, SHOOT IT!!