I don't know how many upgraded cards you've got, but most good upgraded decks can own bronze arena pretty hard, and you can get unupped shards there.
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Something like the monodeath poison above rush does quite well. When the newer shards came out, I used something similar to that against bronze, and it won virtually 100% of games played; I didn't keep stats, but I'll bet it was something like 98% win rate in bronze at the time, no joke, as I'd frequently win multiple bronze special spins in a row without losing one game. (I'm not sure on the exact count of dragons and bone walls and quanta, but it was pretty close to that deck list) As I won shards of sacrifice, I squeezed those in as well instead of some of the walls or dragons or whatever.
As for nymphs, the only ways to get those are from lucky oracle spins, or as prizes for tournaments/leagues. Often, though, people who win tournaments or leagues use the reward codes to get the (even rarer) mark cards; however, since nymphs are usually better in a deck, people who aren't anally retentive collectors will take nymphs instead. I've chosen mark cards myself when I'm lucky enough to get prizes like that, due to rarity, but that's the only other way to get nymphs. Any nymphs in decks will show up as relics instead in spins.