This is one of the easiest ones to complete in my opinion, because you get unlimited continues. Start the game up and just keep beating the levels, eating ghosts when you can along the way, but never having to care about dying because you just insert credits and continue right where you left off.
There are 23 levels with a final boss, and you get 100,000 points just for beating the final boss so that takes care of all the score missions. You should also have completed every other mission except for one along the way just by playing. The final mission will be to complete 30 rounds, so just start the game up a 2nd time and beat 7 levels and then let yourself game over and you have finished this game off!