Ruby Storm has very few creatures worth killing with removal, so Cast Down is mostly dead weight. Its only real targets are the small cost-reducers, Goblin Anarchomancer and Thornscape Familiar, and Snuff Out already covers those. Blue Elemental Blast is the key sideboard card. Desperate Ritual, Seething Song, Manamorphose, Big Score and Seize the Storm are all red, so for a single blue mana it can counter any of them. It can also destroy a Goblin Anarchomancer, which is red and green. Thornscape Familiar is green only, so it is not a legal target. Keep Counterspell to fight over Seize the Storm, and keep Dispel for their instants such as the rituals, Manamorphose and Big Score. Dispel cannot hit the sorcery Seize the Storm. Spellstutter Sprite is fine against cheap spells, but it will not stop a five-mana Seize the Storm. Hold Snuff Out for Goblin Anarchomancer or Thornscape Familiar if they stick early. The matchup is won by disrupting the ritual chain before it reaches critical mass, not by killing small engine creatures after the fact.