Here is a score farming method that can be done on the Primitive Age stage.
As long as the midboss is never touched you can stay here until you game over.
Ideally you want the stage to appear as the 2nd. It's RNG what stages you get so just reset if you don't get the stage after the first boss.
I don't take credit for finding this. It was shown to me but I can't remember by whom. Credit goes to them.