UR Aggro relies on cheap cantrips (Opt, Sleight of Hand, Flow State) and small creatures like Slickshot Show-Off and Elusive Otter. Sunspine Lynx is a sturdy 5/4 body that their small creatures struggle to attack into. Its enter trigger deals damage to each player equal to the number of nonbasic lands they control, so their nonbasic-heavy mana (Riverpyre Verge, Spirebluff Canal, Steam Vents) gives them a real life-loss cost. It also deals damage to us for our own nonbasics, so sequence it with your life total in mind. Thought-Stalker Warlock's menace lets it pressure through their small blockers, and its enter trigger makes them discard a card, which is best when they have already lost life this turn so you pick the card yourself. Stinging Vitriol is a slow sorcery that can't answer their creatures, so it is the weaker card here, and Valley Flamecaller matters less since this is a straightforward damage race rather than a value grind. Race aggressively and use Burst Lightning on their early creatures to blunt their tempo.