Sylvan Safekeeper protects your threats from Swords to Plowshares, which is their only real removal spell, and lets you keep attacking through their mana-elf-fueled board. Stifle has very few good targets here since Survival Elves' key cards like Swords to Plowshares, Armageddon, and their creatures' triggered abilities (Deranged Hermit's token creation, Yavimaya Elder's death trigger) are mostly one-shot effects where countering doesn't swing the game much, aside from an occasional Quirion Ranger untap. Focus on racing with reanimated threats before their Gaea's Cradle-fueled mana advantage and Armageddon lock you out.