In a cocktail shaker, muddle the cucumbers gently with simple syrup until lightly smashed.
Add in the lime juice, gin, and elderflower. Fill the shaker with ice and shake vigorously until well chilled, about 15 seconds.
Fill 2 glasses with ice and strain the cocktail, dividing evenly into the glasses. Top with soda water, then garnish with lime wheels and cucumber ribbons to serve.