I usually do not use missile ships, but if you do then here is a trick that works if a planet has few PD and no ships defending it. Instead of moving your entire fleet of missile ships to the planet, fleet transfer and move them one at a time. As each ship arrives, it moves to just inside missile range and fires off 9 or 10 shots during 30 turns combat (dancing back out of range). Then 2x9 then 3x9 etc. When all ships have arrived pick one to move and tell it to attack again. The whole fleet gets another round and repeat. When the engagement is over, transfer all the ships back into the original fleet of course...

What an exploit! Pound em to pieces!