I found First Casemates: CSS Virginia to be very easy... especially when exploiting it by ramming the enemy monitor. Prioritise armor, bulkheads, and speed. Have the one gun, casemates are not required. Ram the enemy, finish them off. On the other hand, this exploit doesn't work in the scenario where you need to kill a destroyer with a light cruiser because your ship's captain slams on the brakes to get from 30kts to 0kts in a split second so ramming is not possible. If that is actual game mechanic (not allowing ramming), perhaps that should become available after a failed attempt.