My tip: The only real ranged weapon you need is the sitgun, and get a Sldge Hammer for melee. Devestates most enemies in one hit. Then, buy good armour, and have a Penguin Man in your roster...
Tip for conquering cities: Get biggest ship, hire people with maximum attack/defense, and equip them with Claymores/Mythril. Then, attack cities with lower population and bad technology (Old world, Archaic, Rudimentary etc). DO NOT ATTACK FUTURISTIC CITIES.