Chapter 360 : The Demon Is Drawn
The reason that Sato believed that their defeat was approaching was simply because the Kobold King was that strong. Sato alone couldn't compare to it in a one-on-one battle as he needed the assistance of both Xue Yan and Seiichi to intercept the BOSS at critical moments. However, no matter how coordinated the trio was with their attacks, a 3 on 1 battle would be more difficult than a 1 on 1.
In a one-on-one battle, the combatant would only need to focus on his opponent and nothing more. All his moves would be to either attack or defend and they were all performed for himself/herself. But in a 3 on 1 battle, it was different, especially when the adversary was equal in strength to the combination of the 3.