Gruul Goblins is a fast, low-to-the-ground aggro deck full of 1/1s and 2/2s like Goblin Lackey, Mogg Fanatic, and the tokens from Siege-Gang Commander, so Pyroclasm's 2 damage to each creature sweeps their board cheaply while rarely costing us a relevant permanent. Xantid Swarm buys a full turn of insulation from their Lightning Bolt-style interaction and lets us assemble the Hermit Druid combo or set up Survival of the Fittest without fear of a counter-disruption spell on the crucial turn. Duress is weaker here since Gruul's hand is mostly creatures and burn rather than the noncreature answers it's built to strip, so one copy comes out; Elvish Spirit Guide and Wall of Roots are cut since we need fewer early blockers/mana sources and more interaction against a deck that wants to race us before Survival can take over. Treva's Ruins is a slow, fragile mana source against a deck with Rishadan Port and Wasteland, so it makes way for a Pyroclasm.