Grixis Affinity's whole plan depends on cheap and free artifacts — Ichor Wellspring, the Bridges, Myr Enforcer, Refurbished Familiar — so Ancient Grudge is your premier answer, destroying an artifact for just two mana and recurring for a single {G} via flashback. Note that the Bridges are indestructible, so focus Ancient Grudge on their value pieces — Ichor Wellspring, Blood Fountain, Myr Enforcer, and Refurbished Familiar. Red Elemental Blast pulls double duty, countering Thoughtcast to deny their card draw or destroying Utrom Monitor before it threatens in the air. Trim the clunkier ramp pieces since this matchup is a race where you need efficient interaction over mana development. Prioritize using your removal on Myr Enforcer and Refurbished Familiar before they hit the board for free, and keep Journey to Nowhere for their biggest threat.