A few tips
I was having alot of trouble taking down the destroyers myself, however I found that I could 1-on-1 them by sticking far away and keeping them in the corner of the screen where the missles can't hit me. Another tactic can be repeated flybys, but that's riskier.
This is an amazing game, it's simple, addictive, and fun. The only thing I might suggest is that you maybe stretch the first five levels into about ten, with just fighters until 3, then putting in destroyers, putting in more forces at six, and finally including the space station at ten. Other than that, I can't find a thing wrong with the game.