Survival Elves plays no blue spells or permanents, so Pyroblast and Red Elemental Blast have no legal targets and stay in the sideboard. Naturalize does have legal targets here, since it can destroy an artifact or enchantment such as Survival of the Fittest itself, Sylvan Library, or any colorless artifact they might bring in, denying their key engine piece. Mogg Fanatic is cut since its 1 damage rarely trades favorably against their 1/1 mana dorks that are already doing their job by the time they're tapped. This matchup is a race where Armageddon can wreck either side, so try to be the beatdown and close the game out before they can leverage Survival of the Fittest into a long-game advantage.