Izzet Phoenix's entire game plan revolves around Arclight Phoenix returning from the graveyard after casting three instants/sorceries, so Rest in Peace completely shuts off that recursion and is one of the best possible sideboard cards in this matchup. Field of Ruin and Cleansing Wildfire are weak since this deck isn't land-dependent and needs to be attacked on the graveyard axis instead. Beza, the Bounding Spring is a sturdy blocker and life gain source against their small creatures like Picklock Prankster while replenishing your hand. Bring in Rest in Peace before they can build up graveyard resources, ideally pre-combat on an early turn.