Boros Energy plays Blood Moon and Fable of the Mirror-Breaker, and Mystical Dispute can target either of them. It counters any spell unless its controller pays {3}, and only the {2} cost reduction depends on the target being blue. Since none of their spells are blue, it costs the full three mana here. It is still a reasonable soft counter on a turn when they tap out for a key spell such as Blood Moon, Fable, or Ranger-Captain of Eos. It cannot counter abilities, so it does nothing against Ranger-Captain's sacrifice ability. That ability stops you from casting noncreature spells for the turn, so sequence your combo around it. The extra Pact of Negation gives more protection for your combo turn against Galvanic Discharge or Static Prison on a key piece. Nature's Claim stays in because it has real targets here: Blood Moon (which wrecks your nonbasic lands), Fable of the Mirror-Breaker, The Legend of Roku, Static Prison, and Goblin Bombardment. The cuts are Xenagos, which is rarely a creature without heavy devotion, and the slower top-end cards (Wistfulness, Griselbrand, and one Ghalta) that don't help you race their aggressive clock.