Clear Game Unlockables
Clear the game once on any difficulty to unlock:
Very Hard Difficulty
Survival Mode
Special Mission Mode
Adjutant Voice Select (training only)
Enemy Layout Select (training only)

Fast Freaking Research
Research is based on the amount of times a stage is cleared in the story or
training modes. Although you can do this stage-farming trick with any stage, the
easiest (and fastest) one is the 2nd training stage.

After you escape Japan with (or without) Admiral Tsukagi, you can speedily
research your tech tree by repeatedly finishing that 2nd training stage with
a fast (60 knots or better) ship, since you only need to destroy the 3 radar
stations and escape to the south map.

Alternatively, if you're inclined for even faster finishes, you can repeatedly
destroy the Archeopteryx with a ship completely devoted to anti-air combat on
automatic mode (on Very Easy or Easy). You can even skip picking up the parts
from the boss and withdraw from combat immediately after the boss is destroyed.