20 Jul 2017 25 Jul 2017
Change the game settings to lower the difficulty to 1 and give yourself however many lives you think you may need. Just remember that you will have to run out of lives in order to record your score for another achievement. 30 to 50 should be plenty.
Maybe somebody can verify if your score resets if you use a continue. I didn't try. If that's the case, then you can just run with the default lives and continue however many times you need.
Confirmed by Eifel Reaper in the comments that continuing resets your score. He also suggested milking bosses for points by slowly chipping away at them rather than burning them out with bombs so you don't run into a situation where you complete the game without reaching 1.5 million.
I hit 1,500,000 somewhere in level 7 and had died about 20 times. But I did this after Caravan and High Score modes so I had the first few levels pretty well figured out.