Complete both primary and secondary tasks during each mission, and destroy all ships and aircraft in the area before you withdraw. Collect all floating crates that contain bonus items. If the mission is accomplished before you have destroyed everything, do not select Withdraw. Stay on the level until everything floating and flying is gone, then withdraw using the options screen. More often than not, you will receive an S rating and get special access to weapons, hulls, etc.
Breac Wave Gun
After completing the game and unlocking Special Mission mode, change the difficulty setting to Very Easy until you reach the Defeat The Master Of Terror mission (final mission). Complete the mission on the Normal difficulty setting to get the Breac Wave Gun. This gun can destroy an entire fleet and most Superweapons with one shot. Play all special missions multiple times on the Normal difficulty setting to receive rare bonus items.
Completion bonuses
Complete the game to unlock the Very Hard difficulty setting, Survival mode, Special Mission mode, Adjutant Voice Select in Training mode, and Enemy Layout Select in Training mode.
Enemy Layout Select (training only)
Clear the game once on any difficulty.
Firepower
As soon as the revolving guns are available, place as many on the ships hull as possible. It makes ship to ship combat much easier. The average destroyer can hold up to five of the 57MM on deck. Do not worry about the traditional cannons, as their rate of fire is too slow. The revolving guns are very powerful and rarely run out of ammunition. The only downside is that you must be fairly close for them to be effective. Race up next to enemy ships and shoot them and they will be destroyed quickly and generally cannot fire back due to the close range. Upgrade the size/caliber of the revolving guns whenever possible in the R&D section. Once the wave guns are available, add one or two to take out entire fleets, then use the revolving guns to take out the survivors. A combination of wave gun and revolving guns can take out anything in the game.
Gravity Shield
Sink 100 Super Ships to unlock the Gravity Shield.
R&D Section
Research engines, artillery (guns), systems, and hulls in that order. Save your money and spend it only on the most powerful items you can afford or that are available for what you are researching. After just a few missions, you should be able to afford researching battleship and battlecarrier hulls. Remember to also research engines, turbines, smokestacks, and bridges for the hulls you are researching.
Rubber Ducky ASW tech
Beat the game two times on any difficulty.
Speed
Try adding as many boilers and turbines in the early stages for maximum speed. Do not forget to add smokestacks to vent all those boilers. In the R&D section, use your money to research the most powerful engines and turbines available for the hull you have. For the first third of the game, do not bother adding armor as it will only slow you down. The faster you can move, the harder it is for opponents to hit you.
Superships
The best strategy to destroying Superships is to try to match their speed and get up right next to them. Then, attack them with the revolving guns. Most of the Supership's weapons are long range, and they cannot lock on and fire if you are directly beside them. Do not fire from long range unless you are using a wave gun.
Unlock Special Mission Mode
Clear the game once on any difficulty.
Unlock Survival Mode
Clear the game once on any difficulty.
Unlock Twin Hull Battleship
To unlock the Twin-Hull Battleship beast, you must get an S-rank on mission PAC-B-593. This mission however can only be accessed if you are under Braun's adjunct. This can be done in training mode.