There usually is a trick to these and for some it is hard to find. I did not 'solve' this one yet myself and the same for several other and missions in the campaigns in the two first Cossacks parts (EW & AoW). One day I will give up and use a walk-through. Try many differnt things and start over again and again and.. or search for a walk-through or hope someone here will ask you. I think some missions are just hopeless on the highest difficulty.

OK, I started up the game and played through it quickly on easy, something I never usually do but to get an idea of a strategy it works. What I did, successfully, was forgetting about anything but the fleet and the economy to support it; meaning no upgrades for anything except things relevant to the economy or the ships or artillery. Meanwhile I defended the white ship bringing me resources and attacked the right side enemy with my fleet and letting my right sided ally do the land combat part. When they were defeated, I just moved over to the left side and let a single stationary ship take care of defending.

Now on hard I expect you at least need to buy the building HP upgrades too and send a smaller part of your fleet to help defend your left sided ally survive until you can move your main fleet over to that side. They might not last through the whole game but should keep the buggers occupied hopefully long enough. Also, remember to build new war ships.

Why right side first? Because that is where the white ship sails bringing you resources.