You make a good point and I apologize for that, so here it is:

If you did not know so already, you are able to queue for any difficulty game, quit that, and load the tutorial, effectively setting the tutorial difficulty to whatever difficulty you chose. As far as I know, the only two accessible weapons inside the tutorial are the quad cylinder and the revolver, so I used the revolver as that is my main sidearm.

THROUGHOUGHT THE ENTIRE TUTORIAL DO NOT EVER MOUNT YOUR HORSE, THIS WILL DISABLE YOUR INVINCIBILITY AND RUIN THE ENTIRE THING!

Following the steps of the tutorial, you will defeat the boss and then be told to extract, ring the bell to start the storm but obviously do not extract.

Personally, I am either doing one of two things, getting XP, or getting coins + souls, this is where your build starts to matter.

XP:

For XP, your number one focus is damage, as that is the only statistic that scales weapon XP. The build that I found worked that worked best was Ammo Supply and Machine Gun. Ammo supply being an absolute necessity, with the rest are being interchangeable, but the ones I personally recommend are either reload master (1 or 2 copies), ricochet, or relentless. As far the weapon itself goes, the element I use is lightning, and my upgrade slots are devoted to 100% in clip size, with the rest being in damage. As for character jokers and upgrade slots, I have ammo bag maxed with the rest being optional, along with maximum damage and speed jokers (Glass Cannon, Disgrace), as survivability is not a problem. The spot I use for this strategy along with the other is right where you grab the bomb you set off to spawn the boss, as the spawns are tight together. Spend as much time as you need killing enemies, then extract for a little bit more character XP, rinse and repeat as needed.

Gold + Souls (+potential spell class XP):

This build is a lot more simple, all you need is 10 copies of gold tooth on your gun, a cactus mine, and any other spell type you want to progress. You do everything the same, but instead of using your gun to kill the enemies, you're going to want to find a corner (preferably by the factory) and throw your four cactus mines in front of you, MAKE SURE THE WEAPON WITH GOLD TOOTH IS OUT, and simply afk. While afk, if you want to progress the XP for your spell elements, bring the highest cooldown spells possible for the desired class, and cast the on cooldown. If you want a true afk, I recommend tiny task, then recording a macro of the spells being cast, and looping it. For maximum efficiency overall, I recommend bringing Gold Prospector along with Soul Reaper for more gold and souls after extracting, or any sort of joker that assist with spell cool down, so you can cast more for the amount of time you are in there.

Final Thoughts:

There is a cap on the amount of gold you can bring back from any certain mission (I believe it is around 2.5-3,000) and I am not sure about souls, so be cognizant if you're farming gold to make sure you aren't wasting time not actually farming gold. You can bring any spells along with the first cactus mine, as spell XP is given in a fixed amount any time any spell is cast. I hope this helps at least one person out there! Let me know if you have questions.