Jeskai Ephemerate relies on value creatures like Archaeomancer (1/2), Mulldrifter (2/2), Augur of Bolas (1/3), and Murmuring Mystic (1/5), plus the Bird Illusion tokens the Mystic makes. Breath Weapon's 2 damage can pick off Archaeomancer, Mulldrifter, and any Bird Illusion tokens, but it will not kill Augur of Bolas or Murmuring Mystic, so don't count on it to clear the whole board. Avenging Hunter is a Dragon and is unaffected, and your bigger creatures survive it too. Be mindful of your own Arbor Elves and Eldrazi Spawn tokens, which die to it. Malevolent Rumble is too slow against their tempo-oriented draw-go shell backed by Counterspell, and it doesn't add pressure. Keep in mind that Dispel can answer an instant like Breath Weapon. Watch out for Skred, which scales with their snow permanents and can threaten your bigger creatures, so sequence your biggest threats carefully. Keep your core mana ramp and cascade threats to apply pressure before they assemble their Ephemerate value engine.