So, after playing on and off for a couple of months, I've found a strategy that I'm finding somewhat successful, although since it relies a fair bit on luck (being that you need to acquire Ccoa), it probably goes against the intended gameplay of adapting to your situation.
Regardless, although I (in a completely self-contradictory way) do not support a premeditated strategy of exactly what to do, if you personally are having a pretty ambivalent early-mid run or just don't know what to do, and if all the stars are aligned in your favor, you could try this. I should note though, in probably over 20 runs, I've only made Codex progress with this strategy once or twice... Now, a little backstory!
Alright, so I've already found out that vile and dark demons are my absolute bane no matter what build I try (out of those I've tried at least). All the status effects of vile overworking my healers make it really tough when you and demons are near constantly infected. This is incredibly bad when coupled with MaxHP effects (granted, I don't know if any purely demon encounter holds both effects, but still both really scare me). I though of a way around this, though: always get plenty of resistance to dark and body. I was originally planning to start this early off by fusing an abbey lubber onto an easg saint or something of that nature, but then I ran into something I'm surprised I didn't notice before: You can't fuse a demon such that it'll lose all weaknesses. And so, I was like "what? awwww..." because, although not nearly as scary as dark and vile, I've died to light a fair share too, and I don't want to be weak to that either. And ice isn't really much better.
So I thought, why don't I just make MYSELF as resistant to these three things as possible? And thus I searched the Codex, and found the holy grail (pun intended): Aithir.
Aithir (for those who don't know (like me a few days ago)) has full immunity to body, light, AND dark. It's like a dream come true. The only problem is it's weak to fire, ice, and electricity, and that's absolutely horrifying. So, at least in the demons I've captured (not gonna spoil the late-game ones for myself before I see them), I have found a single solution to this problem: Ccoa's Stormy modifier grants resistance to both ice and electricity.
So, I was set: in my first run of the strat., I grabbed Titan's Fist, grabbed slash and body, and began dumping all points into vitality and strength so I could steamroll anything late game with sure slice, and began preparing my ultimate masterpiece. Surprisingly, I don't even think I had a strong ranged attacker, but I actually got Ccoa to my side (might have been an offering...). Once I reached anomaly, I found an aithir, got that too, and using the sweet fusion reduction cost, I grabbed myself a Stormy Aithir for not too expensive a cost, with weakness to only fire, resistance to ice and electricity, and immunity to dark, light, AND body. I was feeling pretty good about myself. And then I saw the soul armor cost: upwards of 16,000 Credits. I didn't come even halfway yet by the time I was annihilated by an Akateko swarm ~floor 10. So yeah, that's my strat. Good luck to any who try it, because it's absolutely bonkers. I don't even really know why I drew this story out so long. Maybe it's because I don't want to do my English discussion questions. Maybe because I've been desiring to share this with someone. Maybe because I love to hear my voice so much I need an excuse to hear it in my head. Probably all three.
TLDR/Summary for people who did read: Get the soul armor transmutation, get Ccoa, and get an Aithir. Fuse Ccoa onto Aithir and use Aithir's soul as armor for yourself. You are now immune to body, light, and dark, and resistant to ice and electricity, with only fire to worry about. You need an unearthly amount of credits though, and need to not die before you can manage this. As a disclaimer, I've tried this far too many times and never succeeded, and only make any actual Codex progress a couple times. Try it if you want though.