Eldrazi Tron wants to land Chalice of the Void, Trinisphere, or a giant Eldrazi threat like Emrakul or a delirious Devourer of Destiny, so extra Unholy Heat gives you efficient removal that scales with delirium against their big bodies. Spell Pierce is crucial to stop a turn-3 Karn or Ugin before it takes over the game, since they're often tapped out early. Surgical Extraction can strip a Kozilek's Command or key answer once you know what's coming. Cut Mishra's Bauble since it's a liability under Chalice of the Void, and trim Lava Dart since it's too small to matter against their large threats.