LASERPITIUM

This achievement is the make it or break it for completion. In order to reach this score, you have two choices:

1) Play Full game using ship 1 or ship 2 on very easy or higher difficulty.

2) Play Full game using ship 3 or hard or extreme difficulty.

If you decide to take the lowest possible difficulty, you need to get as many bonus items as possible and also for method 1, "farm" points by shooting at missiles. The achievement will unlock around the final level or the level before. Best place is the Helos boss that shoots 4 or 6 homing missiles every once in a while. You can shoot this to get points. Ship 3 can't target missiles, so ship 3 has no chance to use this method.

Each difficulty adds about 400-500k of score, so for the most comfortable 3 million but not too overly difficult, my recommendation is ship 2 on easy difficulty.

Here is very easy, ship 2 playthrough where I achieve a little over 3 million for your reference:

If you are looking for specific levels, timestamps are in the pinned comment.