Of course, you'll want to replay Sephiroth to do it all again for a second, third, fourth, fifth, or sixth emblem and make every death hurt as much as possible
Doubling Season is a great example of this
Five mana Time Walks are pretty fair in the cube
Ramp like Sol Ring, Arcane Signet, Boros Signet, and the Mardu Talismans make sure you always have the mana to drop threats fast and recover after board wipes
Well note this for later, as it demonstrates the on-going challenges of Duel Decks as a concept
Theres a small amount of token creature generation and trigger copying present as well, but not enough to focus on as your game plan