The Better Part of Valor:
A little known secret about Cloud Soldier is that you can hit the Escape key at any time during a mission and run away with all of your credits intact!
Use this to your advantage to gather credits, buy a new airplane, then upgrade it early on - this makes all the difference!
Learn from the Boss:
Each boss at the end of the level has a simple pattern it repeats - learn the pattern, take your time, and the boss will fall!