Boros Energy is an aggressive creature deck full of small, efficient threats (Ragavan, Ocelot Pride, Guide of Souls, Fable of the Mirror-Breaker tokens), so Dismember's -5/-5 cleanly answers nearly everything they play. Vexing Bauble shuts off their free spells like Mutagenic Growth-style tricks they might bring in and taxes any zero-cost effects, while also being a cheap artifact we can sacrifice later for value. Summoner's Pact is too slow and risky against a deck full of cheap removal and disruption that can punish the delayed payment, and Aftermath Analyst/Cultivator Colossus are too clunky against this clock. Prioritize surviving the early turns and closing with Primeval Titan or Scapeshift before their Blood Moon-free beatdown overwhelms us.