The game settings are balanced to meet the goals of different players. If you want to test your skills and strategy, you can play the difficult level. If you are just playing for fun, then choose easy or medium level.
Greater variety of weaponry and landscape types gives you huge freedom in choosing your battle tactics. Choose the right weapon type and position to make sure your defense is effective. Airstrikes and the possibility of temporarily increasing weapon power can give you the advantage, and guarantee you will never get bored in the battle process.