How the hell do I finish a run?
I get past the 4th area, kind of easily, but the 5th and the final boss is cancer. Guides recommend getting a +6 plate armor. How the fuck do I get through the game if I don't find it. What if I find it in the second or third area? There are not nearly enough scrolls of upgrade to get that high.
What am I supposed to do? I am missing something but I don't know what.
Specifically addressing your questions on upgrades, since that's what you asked about; here are lots of tips and tricks, but it sounds like you've found a guide that should have many of those. Sorry if you know some of these tips - just laying a few things out there.
There are 15 scrolls of upgrade in the game, three per area. IMO, scale armor can be fine for a win, particularly if you find it already upgraded. As a quick check to see if unidentified equipment is upgraded, I like to throw it on explosive traps - if it's upgraded, it won't be destroyed; if it is destroyed, you haven't lost much.
While I prefer to save most of my SoU for my end gear, it's not totally necessary. If you're using warrior, you can put an upgrade immediately on armor with the seal on it - you'll keep that upgrade through the game, which can be a great help in the early game. Ditto for mage staff - give it at least upgrade early, but it's totally viable to put more to it if you can't find a good weapon early.
If you're feeling frustrated, there's no shame in taking a break for a while - many players (myself included) take >100 runs before their first win.
Random tip: Use Alchemy to create 15 uses of Recycle then recycle your bad potions into Potions of Healing. Sometimes you get lucky and get like 6 potions of healing doing this. Other times you only get 1 or 2.
If you find another Scroll of Transmutation, do it again. I once ended a run with 26 potions of healing.
Other than that, the goal of the game is to learn how to beat it. It's meant to be very hard. I've only won 26 out of 136 runs, though a lot of those deaths were from trying to beat the game with 3 or more challenges enabled. However hard you think the game is, people beat it with even 6 challenges enabled, and many of those challenges are insane. But there are a million tricks to learn for a million situations.
Another random tip: Never step onto a fountain of healing. Instead, drink whatever is in your Waterskin then throw the Waterskin on the fountain. It completely fills it! Now you have instantly have enough dew drops to bless an Ankh.
But did you know the fountain of healing uncurses any equipped items, fills hunger, and (I think) cures debuffs? When I find a fountain of healing, I like to equip my unidentified items (even ones that are too heavy) until I've either found them to be uncursed, or have them stuck. Afterwards you'll know that most of your equipment is not cursed, and get some free curse removal.
Ha! Nope! Nice to know there are still things I didn't know in the game.
Go gladiator and upgrade ghost armor to +15 GG (or any random scale/plate)
Or imbue living earth as battlemage. Or WoRegrowth +10 for infinite seeds and golden lotus as Warden. Or go assassin with hard hitting weapon and one shot everything.
You can also watch some guides on youtube, for example running from cake has a good beginner guide.
I've been playing for years and only won once by using a corrupting weapon combined with a massively upgraded ring of arcana so that just about everything I attacked turned into a corrupted ally and fought for me.
I just put all upgrades on my staff. Rely on finding a bunch of good wands, long range melee weapon and artifacts. Doing a lot of alchemy to get scrolls of divination, scroll of metamorphosis and potion of mastery. Divination only to not waste any of the latter. I use blacksmith on the highest tier armor I can find and star blandfruits to reach the last level.
That light wand is nice for the last area and dark levels. I always try to stand in doors when I fight the eye enemies and the charm enemy can still be hit by wands.
I never made it to the surface, but beating the last boss works.
I also heavily encourage stamina potions, elixirs of dragons breath and toxic essence, and challenge scrolls. Make sure to kill all demon spawners on previous floors. If you have good offensive wands, make stones of flock and shock. Throw flock (trap enemy if possible), throw shock at them to fully recharge your wands
You should use your resources to win. If you lose with SoU in your inventory then you wasted resources and a run.
Replace SoU with heal potions and I'd agree with you. But if you need to overspend SoU to survive the early game, then it was a doomed run anyway.
The Dwarven City and Demon Halls are balanced with the expectation that you'll have heavily upgraded high-tier gear. Sewers and Prison are not. Generally I try to spend no more than 1 scroll per region on low- or mid-tier gear, and save the other 2 for my endgame gear.