Diablo's combat prowess is immense, earning him the status of Rimuru Tempest's strongest subordinate. While his raw power and skills are lower than Rimuru's, he is able to compensate for this with his battle sense, techniques, experience, and deep knowledge about magic and skills.
After evolving into a Devil Lord, Demon Slime Rimuru felt like Diablo was no longer an opponent they can beat. Due to Food Chain, Rimuru's power grows as their subordinates' powers grow on top of being able to access their subordinates skills, making them virtually impossible to lose against one of their subordinates.