Even defending subs, if they are undetected, fire in the opening phase and casaulties are immediately removed. So yes, they could sink the BB before it even gets a slot.
As I understand it, you cannot attack subs unless you detect them. Therefore, without a destroyer, they will get opening shots. After that, the sub’s player could chose to remain in battle which makes them detected and eligible for attack from air. Of course, I cannot imagine anyone doing that since AFAIK subs cannot attack aircraft.