Boros Energy gets value from Seasoned Pyromancer, whose card can be used again from the graveyard, and from Fable of the Mirror-Breaker's discard chapter. Leyline of the Void exiles their cards that would go to the graveyard, so it shuts off that Pyromancer ability. It only affects cards going to an opponent's graveyard, so our own Unearth and Abhorrent Oculus synergy is unaffected. Toxic Deluge is a sweeper that answers their wide board of 1/1 and 2/1 tokens from Ocelot Pride, Ajani, Nacatl Pariah, Voice of Victory, and Seasoned Pyromancer for a small life payment. Harbinger of the Seas blocks well against their small creatures and turns their nonbasic lands, fetchlands included, into Islands. It does the same to our own nonbasic lands, so cast it with care. Spell Pierce and Force of Negation are weak here since Boros is a low-curve, creature-heavy aggro deck, and Theorix Charm's counter mode is similarly low-impact against their creature-based plan.