Gruul Delirium fills the board with aggressive, protected threats like Patchwork Beastie and Wildfire Wickerfolk, using Blossoming Defense and Rapid Rescue to dodge removal, so you want sweeper effects to answer multiple threats at once. Slagstorm punishes them for going wide, while Sunspine Lynx and Sear give you efficient ways to keep pace on the ground and in the air. Spell Pierce is weak since their deck is almost entirely creatures and combat tricks rather than impactful spells worth countering, and Multiversal Passage is too slow against their low curve. Watch out for Blossoming Defense protecting their key threats from your removal, and try to play around it when possible.