Changing the lives in the game to the max allowed andchanging the mode to hard (I believe it gives you more points, not sure) and just playing the game seemed to get me this reasonably fast. Avoid being shot, shoot other guys (just keep shooting). The same should be done for the mini boss.
There'll be a point where the game turns into a top down shooter, and your score goes away, just keep playing (I flew to the left side and that seemed to work). You'll likely get this a short way into the top down portion.