Storm Fortress is extremely overpowered. She has a bazillion cannons, shoots flaming mortar, and maneuvers more quickly than you, even though she is a much bigger ship. I died almost instantly in my first two tries. Then I noticed something. The Storm Fortress has a huge weakness. She doesn't carry Burning Oil Barrels. This means that, if you manage to keep your ship right in Storm Fortress' butt, she can never harm you. It took me two more tries and I got her. This isn't very hard, if you know how to exploit certain gameplay mechanics.

If you don't know how to Ram Charge yet, it's very simple. While aiming down the Front Cannons, press the Speed Up button to Ram Charge. Your ship will then run faster than its normal top speed for a short uninterruptible period of time. Use this to get out of enemy kill zones. Also, remember that it has a cooldown.

As soon as the battle starts, my suggestion is to let your allies die. They don't do anything other than get in your way. I kept a distance of 800m from the target until all my allies died. If you're not that patient, you may attack them a few times with the Mortar. Feel free to ignore this part, if you're good enough to avoid bumping into your allies during the battle.

When you're ready to engage the Storm Fortress, make sure you're aware of her bearing and go straight for her butt. This is the most difficult part. You'll probably lose a block of health just trying to get close to her. That's fine. Use Ram Charge whenever it's available in order to close the distance as quickly as possible. When you manage to get behind her, turn to face her butt.

Maintain a short distance, or she'll shoot you with the Mortar. Whenever she tries to turn around to shoot you with the Broadside Cannons, just turn to the opposite direction and Ram Charge, then turn back to face her butt. Your first priority is to always be behind her. If you do that, she won't harm you again.

Whenever you're behind her, just spam the Puckle Gun and the Explosive Shot (that's the Front Cannons). Keep shooting at her butt at all times. Sometimes, you may even get a chance to shoot her butt with your Broadside Cannons while she's turning, right before or after Ram Charging away. But rest assured, the Puckle Gun and the Explosive Shot are enough.

When the Storm Fortress' health is low, two much weaker Man o' Wars will spawn. Don't panic! There's plenty of time to finish her off. You have two choices. Either stay the course and keep spamming your attacks, or run away and finish the Storm Fortress off with your Mortar. Either way, ignore the enemy reinforcements completely until the Storm Fortress has sunk. If you've done this with more than a block of health remaining, you're pretty much safe.

The secondary battle with the Man o' Wars is relatively easy. Just be careful not to get surrounded and not to get rammed by one of them while you're focusing on the other. The easiest way to get rid of them both is to keep running away from them while spamming Burning Oil and focusing your Mortar on one of them. When it's just one ship left, it's a piece of cake. I'm sure by now you know how to deal with one measly Man o' War.