how do you die from cain? you dont really need a bo for him. What I do is:
1.Enter.
2. Immediately cast 2 golems on him (i only have 2)
3. spam maelstorm
4. whenever you see the black crows don't bother attacking - just spam golems until the birds disappear
5. once the birds disappear spam maelstorms again
6. if you see a green tornado coming towards you, just move to the left or to the right to avoid it
I remember that you do much more damage than me (and you probably have more golems as well) so it shouldn't take you more than 15-20 seconds to take him down. I dont use BO for cain (i enter the portal with around 1600 hp) so your health shouldnt be an issue either.
I hope that helps.
P.S. However, I am sure that lampogriz will be able to give you even better advice, since he is the cruci master