It was a large, heavy-built, ground-dwelling, bipedal carnivore that would have been an active feathered predator hunting with the enlarged sickle claw on each second toe. Estimated at in length with a weight about , "Achillobator" is considered to be one of the largest dromaeosaurs, along with "Austroraptor", "Dakotaraptor" and "Utahraptor". "Achillobator" was a deep-bodied and relatively short-armed dromaeosaurid with stocky and robust hindlimbs. Some of the most notable features consisted in the robustly built skeleton—an unusual trait in dromaeosaur dinosaurs, which were generally lightly built animals—such as the deep maxilla and femur, and the primitive pelvis, having a vertically oriented pubis that differs from the rest of dromaeosaurids.