Longbow is a missile cruiser. They are equipped with a heavy long-range missile launcher and a medium anti-air turret. Their main use is getting near the shore, then launching powerful missile barrages against land bases. As long as you can manage to get them close enough, it takes only a few missiles to completely level bases to the ground. Fortunately, their range surpasses all land-based defensive structures, even the Pulsar, so the only viable ways to stop them is to intercept them or hit them with tactical nukes.
A high missile trajectory enables them to shoot over mountains and other natural land barriers, which can stop other ships that use plasma cannons.
Take advantage of the element of surprise by combining them with jammer ships to hide them from enemy radar, and let them sneak in close enough before starting the bombardment.