Messenger is a missile cruiser. They are equipped with a heavy long-range missile launcher and a medium anti-air missile turret. Their focus is typically to get to the shore of an enemy and unleash an overwhelming first strike against their defences. A few missiles will completely level most structures and their range exceeds all land-based defence structures, even Annihilators, so the only realistic way to counteract 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. As their munitions are not plasma based they will also completely ignore plasma shields.
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.