First, take all the weapons and boilers of the initial battleship. Then, buy the cruiser body and put the boilers and weapons on it. This way you can easily farm destroyer levels, maybe even dual destroyers. You can do this with the battleship body for greater results. Even without armour, the bigger your ship is, the stronger it is, and since hull parts are free you can easily make a strong ship by making it with as many hull parts as possible.