In my experience, this game isn't terribly luck dependent. I nearly 100%ed the game prior to the update deleting my save file, and I didn't actually ever lose a run. The camp doesn't heal much, but Ceridwen doesn't even use the camp ever unless she gets really bad luck right before the boss fight, and Gwen can use the extra 6 hp usually but there were only a handful of times I was going into the boss with sub-10 hp. Post-update the new spells being mostly worse does make Gwen runs harder, but I still haven't lost one yet (though I have twice finished on turns mushroom man would have otherwise killed me had I not finished him first).
You're right, though, that the difference between runs is rediculously rng dependent-- getting +mana/turn makes the game a LOT easier, and you get that at least once ~60% of the time it feels like. Gwen runs where you don't get plus mana are HARD, and one of the two almost-deaths was a Gwen run where I got no mana and only 4 spells (the shop was trash and I was dumb and didn't check first). And then a run where Air Blast gets Barrage+Giantslayer, for example, makes Gwen look better even than a bad Ceridwen run. If the game were extended so that the weaker runs didn't all win anyhow, that would be pretty annoying.