Mission Select &Skip
To select the mission you want to play, at the mission-select screen press down on the left analog stick and press "black," "R," the right analog stick, "white" and "L." If you pick a mission and find you don't want to play it, you can skip it. Pause the game and press the left analog stick, "white" twice, the C-pad down button, the right analog stick and "white."
Music and Funny Briefings
At the title screen, you can listen to alternate music by pressing the left analog stick twice, the right analog stick twice and then the "L" button and the "R" button. If you want to listen to humorous mission briefings for missions 1, 3, 6, 8, 9 and 14, press the "L" button twice, "R" twice, the left analog stick and the right analog stick.
Use Missiles
Missiles are the best weapon you have in "Gun Metal." You can lock onto multiple targets at once and destroy the targets with one hit, and missiles help bail you out when you are outnumbered. It also helps that the mech and the jet have separate missile packs, so if you're in a tough spot and run out of missiles in one form, just switch to the other form and attack.
Mobility
You should always be moving in this game to avoid being destroyed in a matter of seconds. The jet is much more mobile than the mech and is more useful for avoiding damage, even though the mech has a shield. The mech's shield will break down and take more damage than the jet, which can evade damage effectively.