GHOST 1.0

This achievement only counts the cubes that you have ON YOU. If you die, you lose all of your cubes and have to earn them again! If you've unlocked the previous achievement for 10,000: then this one won't be much harder to unlock IMO. Below was my strategy for obtaining the 20k needed for the currency achievements.

First: wait until you get to the Chunker section. From there, you can buy a powerup which doubles all drops called "Double Loot", it will set you back 3000 energy cubes but is well worth the investment. The shop where you can buy this is in the very left of the area (very near to the final boss of the area).

Second: With that powerup enabled, now go fight the final boss in the chunker area, after defeating him you will get at least 7000 energy cubes (I don't remember if I bought the powerup before or after I fought the boss lol).

Third: With that hunk of metal junk defeated: you should now backtrack with your completed gold card and traverse the new areas in blue that opened up. Simply from my stimulus from the chunker boss and fighting through the rest of the unlocked blue area I was able to collect all 20,000 energy cubes that you would need for the currency based achievements in this game.

Some tips while going for this:
-Be wary of things that would instakill you (the grinders in the chunker area, lasers that haven't been deactivated, etc).
-Be wary of your health! remember that you can also go to a savepoint and replenish your health there.
-Make manual saves constantly! That way if you do accidentally die: you can just quit (WITHOUT saving) and stop your cubes from being lost forever.
-Remember to use your health regen powerups if you have them!