Time is my jam. Fast spells that negate enemy damage are amazing for breaking up the board and tipping the game in your favor.
Thoughts on Rooted Observer: Even with several stealth units in my deck, I only got its special ability to proc once. That was when it was already late game and I played it immediately in sequence with a stealth unit. If you play it early in the game it seems to always draw removal, so at least it eats up a spell that would otherwise target your higher value units, but otherwise it's probably not worth running.