Start your Voidigo Run (I think the Difficulty doesn´t matter, correct me if it´s wrong)
I recommend using the Character "Drash" because of his speical Ability "Antivoid Combo Strike".
This simplifies the whole run, how exactly will be described later

Challenge the Void (only possible if you unlocked it)

Save your crystals by not buying ammo or HP.
-to save as much ammo as possible:
Before attacking an enemy jump on them so it´s easier to shoot at them.
With the help of the Special Ability "Antivoid Combo Strike", you can also do pretty powerful area damage afterwards. Also take items that bring you something when you jump on opponents (i.e. all kinds of slippers).
If you do enough damage with your jumps, you can even kill bosses without a single shot
-to save as much HP as possible:
Doge attacks. I know it sounds dumb but that's just the way it is. You don't even have to run out of the radius of the enemy's attacks, because you can doge them if you jump in the right moment.
You can also swap an item in the Anitvoid for an extra life so it shouldn't be too difficult to have enough HP
Destroy everything that drops the Crytals (every object that glitters).

Do the same things you did in the 1st floor. Now you don't have to challenge the Void anymore, because you will shop in this floor and you will only get the extra crystals in the Antivoid

The only problem with the whole thing is that you can only unlock the achievement if you get the item "Token of ..."(unfortunately I forgot the name haha), which gives you a 10% sale on the whole shop.
Without that you can just reset the run.