You must upgrade a character to level 30.
Which is a lot.
I recommend repeating Maries Remix which nets 350k ectoplasm in a 25 min run. Put all of upgrade points into. Weapon damage, range,overheating then time extension and increase points.
This will speed up level completion times and maximise points. Remember you can press Y to remove skill points you have already invested.
Once you reach level 10 each character has a special weapon upgrade. It is last box on the screen, this is a massive help.
Remember to also use the controller exploit at the end of each level. As there is Ann achievement for fully upgrading all4 characters.