Leviathan is a battle submarine. They have both unmatched damage and range. The threat they pose to unguarded fleets is not to be underestimated and their ability to deal damage at range makes them truly dangerous. While devastating in the correct circumstances the Leviathan is vulnerable to both submarine killers and torpedo launchers.
The ideal use of them is to destroy heavy ships and protect the sea from T3 amphibious units. Utilize their range and try to keep a good safe distance.