Eldrazi Tron is a colorless big-mana deck with almost no creatures worth killing with Lightning Bolt (Devourer of Destiny and Ulamog are both far too large), so trimming a Bolt for more impactful cards is correct. Obsidian Charmaw comes down cheap against their many colorless-producing lands and can destroy an Urza's Tower, Urza's Mine, or Urza's Power Plant to disrupt their mana base significantly. Nihil Spellbomb offers cheap graveyard interaction. Keep Thoughtseize in to strip Karn, the Great Creator or Ugin, Eye of the Storms before they can take over the game.