Go between her legs.

Just as the two of you were about to meet, you dropped and slid between Arya's legs, coming up behind the large pokemon. Arya seemed suprised, and you knew you would have to act quickly. You tried to wrap your arms around Arya's waist but due to her size it was difficult. Catching on quickly, Arya shut her legs, denying you access to her pussy.

From here you knew you had two options. With her legs close together, Arya's balance was surely compromised. On the other hand, she was a water type, and her naturally lubricated body should make it easy for you to slip into her ass...