Finished my Eld0 run on "first try". In quotations because the only way I got through it was through save-scumming the entire run, so not sure if this still counts as "first try" lol
All the drops have a chance for higher rarity cards, and you can save after beating a mob, and reload infinitely until you get the cards you want. About half way through you will more or less have all the cards you need. Relics are really what you are looking for later on.
You can also just "Retreat" after going through couple levels. This will put you back in the beginning of the zone, but you keep the cards you found (I think also the rare relics too? not too sure).
My deck was mainly Witch cards as I think they have a very powerful combo due to "Tiheera!" (3*) being bonkers OP other than against few monsters that use hand manipulation.
Tiheera! allows you to, at beginning of your turn, discard a card at hand and get another one from graveyard back. This means you can discard debuffs or cards with graveyard effect, and able to reuse other good cards from graveyard. Great when you generate debuffs yourself, or when enemy gives you a lot of dumb cards like Serious Curse. Just toss them all!
Once you are able to beef up your deck with various means (relics, debuffs, transient cards), you will have a decently healthy deck despite most of them being useless debuffs. But since you are using cards from graveyard each turn it really doesnt matter. I had an easier time fighting later boss monsters who like to shove debuff cards in your deck than earlier monsters when I dont have any relics, because your real deck is in your graveyard.
For support cards, I prefer Eros cards like "Evil Sex Toys", which allow you to tutor 2 Witch cards from your deck while adding 2 debuff cards to it (which is actually a positive for you). These debuffs/transient cards are also great for other offensive Witch cards that deal damage base on them. 1-2 recovery cards in case your Tiheera gets sent to graveyard before you find it.
Other combat cards like "One two punch" "Amadzumi - Convallaria" are also good damage choices since they deal damage when sent to graveyard, and are decent too when played from hand.
You can also pair with hand manipulation cards if you are up against oponents that draws only 1-2 card per turn as you have a good selection due to Fresh Familiar and other cards. It's pretty funny when you clear their hand and they just do nothing for one round, and you bring back the card from GY to discard their hand again.
Of course, normally to get all the right cards is near impossible on a normal first time run. At one point I feel like I am seeing the load screen more than the actual fight lol. I dont think its worth the effort and I wouldnt suggest it honestly. Just play/lose normally. You can get Eld0 runs MUCH easier on a NG+ run.
If you want Eld0 runs, you can use one Demon card occasionally to get past some hard boss fights. Just remember to lower your Eld level before using it again (Gaze into mirror, etc).
I do wish there were more scenes tho.........