Tetzimoc, Primal Death is one of my favorites, as is Renata, Called to the Hunt and Master Biomancer, both efficient ways to get counters on your creatures
One card that springs to mind is Ram Through which is a green removal spell that also doubles as a burn spell if you have trampling creatures
Maybe cast this later in the game when you can flash something in to protect it
Muddle the Mixture Muddle the Mixture is a counterspell when you need it and you can transmute it into a tutor for a 2-drop when you want, as well
Karns Bastion Karn's Bastion is usually easy to slot into the mana base of decks with counter themes, even though it doesnt actually get activated often
Those players who brave the dragonstorms and emerge victorious will make it to Day Two, which features Tarkir: Dragonstorm Draft