At the biginning of the game you get to choose which country to play as, American, British, Japanese, or German. My reccomendation is to pick German, even though the description says " FOR ADVANCED PLAYERS". In later levels the advanced German aircraft beats all. And in the fight against Druna Skass, the top German aircraft intercepters and bombers will destroy the ship without you even getting close to the ship. Use these planes on the x-battlecarrier and equip a balanced weaponry ( main weapon, anti-aircraft, and anti-sub) and you should be able to be victorious in any battle.
Easiest way near the biginning of the game is at level a-4. Keep entering and destroy all transports (34 total). Game only allows 999 of each kill. At 999 transports, you will recieve an x-battlecarrier hull. The only other way to get this hull is to pick up as an enemy weapon around c-4 to c-9 after completing the game once.