Can you tweak the config file to give yourself nuclear warheads for your deck gun? I don't see how you're going to win that battle otherwise.
What would be neat is if you could hide behind merchant ships and let the battle wagons sink them for you.
Ages ago I had my boat in a horrible storm that just went on and on. I stayed at depth as much as possible, but had to come up periodically for air & recharging. On one of these trips up, I surfaced just astern of a Rodney class battleship. I could see the screws coming up out of the water, but was so close that they couldn't shoot me - if they saw me at all.
This got me thinking: Do you suppose there is a minimum distance for their secondary armament? What if you surfaced right next to the battleship?