Just do early on speed runs to the exits. use your first crystal on purify and earn some money that way if the weaker enemies have their back to you and you have a decent weapon like an axe or katana. You can usually backshot the legs, floating head (extremely rare on first dungeon), maid and basic zombie chick and purify them without the timer starting. You can also backtrack out the previous door to stop the timer if you get bad rng with a mob spotting you the moment you load into a floor after you take them out.
Your main goal is to find a katana and the double barrel shotgun, those are currently the two best weapons for melee and ranged, all other weapons are very subpar in damage. Just make sure you maintain repair kits via finding them or using extra gold to purchase from the vendor lady. If you let it fully reach zero, the weapon will be destroyed. Second unlock with a crystal should be the losing half if you fail a run so you don't lose all your money.
It gets much easier when you get used to it and you can also get lucky and find additional green shards inside of chests in both dungeons. The game play actually becomes incredibly easy and borderline boring once you get the shotgun and get a firm grasp on the predictability of the mob spawns and map layouts.
Be making thousands a run, and if you can start farming the first boss, she drops a shard every time and a large sum of gold, but it is not intended and will be fixed int he next update as confirmed by the dev. Some of us have mentioned to them to add a way to revisit the first boss fight since that wasn't even on their radar at first. I don't know if the next update will make our current saves incompatible, so if you wanna test all that is available in this version, a few boss kills will make you strong enough to see it all easily.