1) complete the game on normal mode.

This will unlock challenges.
2) Take the gun game challenge.

Gun game makes it so that killing an enemy randomly changes your weapon to that of any selected weapon in your arsenal and maintains your current ammo. You'll see why this is a complete game changer.
3) Refine your gun selection.

Recommended weapons:
Meconopsis 60

BR6

Brianna Model 1900

The BR6 and Mec 60 are both very good workhorse weapons, but, more importantly, they have a ♥♥♥♥ ton of AMMO.

I am purposefully excluding the M2 Dendrocide because it sucks balls in normal mode, but if you can make it work then definitely use it.

The Brianna Shotgun was picked for its high damage per shot and high RPM count.

The idea behind this cheese is that getting a kill with one of the LMGs has a chance to swap your weapon to the Brianna, while also keeping your ammo count. You can rack up as much as 65 Brianna shots like this (using the BR6), which is enough to kill every boss in the game, without drinks necessary.

The exception to this cheese is the final boss. Since you would have spent all your overflowed ammo on the heart of the mansion, the forsaken heart would likely have to be completed as usual. My advice? just dodge a lot and constantly shoot. don't use slow-mo as many of its attacks are easier dodged in normal speed. After a few runs you should already have the hang of the boss, so don't worry.
Here's my sober final boss completion if you get stuck.


P.S this cheese doesn't work with the Blunderbuss because the RPM of the gun is inherently slow