I needed maybe 20 minutes of intense combat to kill this flying beast while using potions, oils (increase attack) and signs (Quen - shield, the most used sign in general, and Aard which worked well, but was needed good timing for taking dragon down during his flying raid). My Geralt is now on lvl 14, this dragon was lvl 21 (in this case it was quite visible difference, one bigger mistake and Geralt was killed). He used acid attacks, sound wave attacks and physical attacks as well including flying raids.