I just finished that misson 30min ago on my first try on hard. I created a castle at the point where the water is smallest to keep out enemy ships and did a fish boom. Taking reds base should be your first priority because they only create Light Cav and Archers so its doable with a small army. As soon as you think its secure enough build the castle. All enemies give up as soon as the castle stands in their city. You can also snipe purple like it says in the tips. As soon as the hero dies they resign aswell. If you want you can also take out blue production buildings. they dont have villagers. After that the odds should be in your favor. Good luck!