go through it twice (If you want diversity in IJN BB choose different bb2 each time)

other than that, I reccomend two bo's to have better supports as you will need to hit the speed cap on your ships to make the grind more bearable.  Or else you won't hit speed cap in a reasonable time like I am with my SY.   

OK thanks for the tip.  Another question, I intend to level up my gunners first on my TW ship so they are ahead of my BO.  What would be a good level to get them too before starting them back on the destroyer with my BB BO?