This is more of a general question than anything: how does one get the cards needes to beat/farm high LV duelists? I'm a relatively new player and I don't have many options to rearrange my deck with. I'm also saving gems for 10K boxes. On that note, how do I get SR card rewards? Even with a high enough duel assessment it always seems up to sheer chance.
This question is like "how do I become successful person in life". It's hard to answer you. Easiest answer is, keep collecting cards whenever you get the opportunity.
Ranked duel, events, gate LD, minority from shop, etc.
We don't get strong in a day. We accumulate our strength over the course of few months (I played for 200+ days).
Pay attention to decks other use. Slowly you will know strength and rating of each cards. Then you start accumulating cards, and subsequently your dueling strength.
Focus on the Bingo and unlocking Ishizu Ishtar to get two copies of "Gravekeeper's Vassal". Open the Ultimate Rising box to get "Secret Pass to the Treasure". Keep beating Mutou Yugi in the Gate once he shows up there, until you get at least 2 copies of "Union Attack".
Farming in this game only works if you have those three cards, no other way has been discovered.