What makes a boss hard?

Not stats and gear: most content now deals little or nothing if you do it right. Difficulty is two sliding scales. Punishing bosses hit hard when you make a mistake. Intense bosses ask for more correct inputs to avoid damage.

A punishing boss can be easy if it isn't intense, and the other way round. When you're learning a new mechanic, intensity matters less than punishment: learn where mistakes won't send you back to Lumbridge.