Bring 1 cup of water to a boil. Stir in sugar until dissolved. Let cool to room temperature. Stir in lemon juice, cranberry juice, lemon zest and tequila. Chill mixture for at least 1 hour or until cold.
Stir pink lemonade mixture again and pour half in a blender along with 4 cups of ice. Mix until slushy. Pour into glasses and garnish with lemon slice. Repeat with remaining mixture and ice to make another batch.