If you have finished the game, it'll be WAY easier with the 3rd ship unlocked (will not say it to not spoil you, but if you're a Cave fan you'll definitely know it ^_^ ), you can even pass the 60M bar before the boss, and your combo counter will raise REALLY high.
If you haven't, you can do that, but you'll have to :
-Keep your combo counter all the way to the mid-boss and then to the stage boss (the GP meter is on the right side of the screen)
-Get ALL three bonuses : No Miss, No bomb and the destruction bonus.
-Get the first (red) ship. The widespread shot will help you keep your combo, and the fact that the shot is less powerful than the second ship can be helpful too to raise your combo and milk some enemies before the next one appear.
With that, that could be it for this pretty easy cheevo. Good luck and grind for the gold ! =)