In a singleton format you have to run a set of cards that all fulfill the same basic function, so instead of running 4 copies of Harmonize, you might run Harmonize, Greater Good, Beast Whisperer, Return of the Wildspeaker, Garruk, Primal Hunter, Shamanic Revelation, Soul's Majesty and Rishkar's Expertise
You'll only have at most 1-2 turns to change the outcome of the game
Day/Night had done a good job of making it easy to remember what state the creatures were in, but everyone involved realized that it wasn't working
While you won't find Arbok's best cards in some of the most expensive Pokemon TCG sets, it has a few appearances in vintage expansions, including Expedition