What Jakub34 tried to say..
1. Kill all shooters in level 1 except 1. Then dodge that last one's bullets and wait for purple ships to come by so you can try to farm extra ships.
2. Start at the right side of the level, move to the left as you go.
3. Try not to stay on the wall all the time or you may get stuck between bullets.
4. Hold the controller in a manner where you can use multiple fingers to press A in case one finger aches.
5. Get a higher score, and possibly get InvaderMan.
Some of my tips..
1. Don't get too excited during the 18000 and 19000's, this always ends up in you losing all your ships, eventually dying.
2. Keep trying, the day I grinded for it, I got a score of about 19200-19700 around 5 or 6 times, but don't give up.