Best way to grind bp other than the battle tower?

Once I beat the first 7 trainers, I only receive 3 bp. When I move onto the second set of 7, I always end up losing. If anyone could help to give me a more efficient way to grind bp other than the battle tower, I'd be very grateful. Alternatively, how could I get farther in the battle tower to earn more bp? Any help at all is greatly appreciated.