Against soraka, Soraka will go to ward alone during the mid-game. You can abuse this by looking to ambush her as she goes to ward an objective. While she is dead, you could force a fight or take an objective with the numbers advantage. 

Invest in an Executioner’s Calling or Morellonomicon if your champion benefits from it. This will reduce Soraka’s healing potential from her W and Ultimate R to help you win the team fight.

As Soraka will be positioned in the backline, it will be difficult for you to take her down fast. To make killing her easier, try to flank from the side rather than running directly towards her in a team fight. Against soraka, You can abuse this by trying to lure her while she's guarding a goal. While she's dead, you can force a fight or take a goal with the advantage of numbers. Invest in an executor, Calling or Morellonomicon if your champion takes advantage of it. This will reduce Soraka, the healing potential of her W and Ultimate R to help you win the team fight. As Soraka will be positioned in the backline, it will be difficult for you to get her down quickly. To kill her more easily, try to flank her side rather than run straight to her in a team fight.