So, I eventually managed to farm the levels between 6 and 9 - with a Vampiric Knuckleduster.
Armed with those, I offset the gradual loss of life due to starving, and I even got a huge boost from the flies swarm - I actually recover some ~20hp every time I fight one of those. Plus, I get Health Potions, and over time I eventually hoard them.
Fighting Skeletons gives me free weapons, and Gnoll Mages leave behind scrolls - and yes, that includes Scrolls of Upgrade.
I suffered a setback when fighting a Gnoll Brute on Lvl9 (?) which killed me and used up my Ankh - but I managed to re-farm 10 potions of Healing, a Longsword and enough Scrolls of Upgrade to make it usable. I then took on the Tengu and died there because I miscalculated the amount of damage he deals with Shurikens and when to use my potions - fair enough, it was my mistake.
Having said that, it is entirely possible to farm everything, from Healing potions to Gold (by selling weapon drops) before bosses this way, by continuously going back to those levels. Even the last boss should be trivial if you bring with you an arbitrary large amount of Healing pots.
Still, this game is so unforgivably hard that, out of exceptional *LUCK*, I see no other way of beating it.