personally i would ignore the bs as much as possible because i am lazy by nature. bs's are slow, got limited arcs and got no cd so if you manage to get the transports to panic and fly off, you're good to chase them down outside of the range of the bs. if they stay close to the bs the transports can end up blocking the guns/arcs which is also pretty nice, and if thats the case i'd kill the bs first because it should be very easy. if you decide to attack the bs, and if you're in scyllas and the jumpship is an aquilon, attack it from above, the heavy guns cant shoot upwards.
its also my impression that most jumpship captains arent exactly pro bs pilots, but of course this cannot always be the case. but i guess its different from situation to situation - if you leave the bs captain alone and he shoots the cargo from the transports you've wasted, you'll know next time to go for the bs first. i guess its a case-by-case thing, and it takes some practice. i've never done it simply because i havent been in the right place at the right time. i've got a cloaked jorm so if you mollies need some additional firepower, ill gladly help out