I've already done a mission where the same problem occured...
a shadow bc pahses in and has to destroy a raider Olympus.... when I made it attack via initial orders, it approached an did not fire... I enabled "All ships beam-freed by default"
1. solution: I placed a waypoint path passing the Olympus and used the event "fire-beam" to ensure the ship becomes hit with the main beam....
2. solution: rotate (x) the bc a while until a so that it has to turn until it can fire....
hint: If you make them attack each other they'll just fly past each other 'caue the shadow has with it's orignial main gun just a very limited angle for firing (dunno why - came to this after a whole lot of experiments)
worked fine with my mission