Damping Sphere is devastating against their Urza-tron manabase, capping Urza's Tower and other multi-mana lands at one mana each and crippling their ability to cast Karn, the Great Creator or Ugin, Eye of the Storms on curve. Counterbalance can lock out a huge portion of their nonland spells since so many of their threats share mana values (Force of Will, Stock Up, Kozilek's Command). Snuff Out and Toxic Deluge are weak here since they have almost no early creatures to kill, just artifacts and planeswalkers, so those come out. Keep Force of Negation and Force of Will in to fight their own countermagic package.