Gruul Sligh aims to kill you with an early creature rush backed by Lava Dart, Lightning Bolt, and Incinerate before you can land and protect a Dreadnought—a resolved Phyrexian Dreadnought ends the race immediately since no amount of burn directly answers a 12/12, so your priority is protecting your Stifle or Vision Charm with a hard counter rather than trading counters on their spells. Hydroblast and Blue Elemental Blast are your premier sideboard tools here: use them to counter burn pointed at your face or to destroy Grim Lavamancer and Jackal Pup, which are their most efficient sources of incremental damage. Powder Keg is a clean sweeper against their early board—leaving it at one counter clears Jackal Pup, Mogg Fanatic, and Seal of Fire, while waiting for two removes Grim Lavamancer and Quirion Dryad before they grow out of hand off your blue spells. Post-board they add Pyroblast and Red Elemental Blast to disrupt your countermagic and threaten your permanents, so be selective about when you tap out for the Dreadnought and keep mana available to protect the combo.