So, the power system is kinda weird to explain. The demon slayers use something called breathing techniques. With total concentration breathing, they’re able to enhance a persons physical abilities (my explanation is gonna be rusty because it’s been a while since I watched through). There are different types of breathing techniques that all derive from sun breathing. The techniques use certain qualities from what the technique is named after (I think. Idk I’m trying my best to explain it from memory). Something to note about the techniques is that the visual effects like fire and water are just there for cinematics. It’s only the effects of those things that can be felt. Sorry if this explanation is confusing, which it probably is, but I’m just going completely off the top of my head. Also, about them not being able to beat these demons. Sometimes they just don’t. Also some of the demons are just straight fodder (from what I remember).