First make sure you have the "Spark Fusion" spell. It can be found in The Abandoned Mines off of the Bandit's hollow (Iron Key is required). Once you have the spell cast it on Ashley's weapon and attack the Elemental using a Blunt or Edged weapon. It can cast the "Aqua Blast" spell so buffing your armour (with Undine Jaspers and Marid Aquamarines) will be helpful.