Dimir Midrange leans on Orcish Bowmasters and Psychic Frog as small, disruptive blockers/attackers, so Pyroclasm can two-for-one their 1-toughness Bowmasters tokens and early Frogs. Skullcrack provides additional reach against their countermagic-heavy plan (Counterspell, Force of Negation), though it is counterable by Spell Snare (given its mana value 2), and punishes any incidental lifegain while dealing 3 direct damage. We cut Shard Volley since sacrificing lands against a deck full of hand disruption and counters is too fragile, and trim a Rift Bolt since casting it off suspend is telegraphed and gives them time to hold up Counterspell. Play around countermagic by sequencing cheap burn first and saving redundant threats for after they're tapped out.