"I have a 5mW laser. If I defocus it to a cm^2 spot, that means I need to get ~25 joules. If I point the 5mW laser on the skin for 5000 seconds, or 83 minutes, that's 25 joules. Perfect!"
No. It doesn't work like that. The mechanism is thermal. The body will take the heat away very quickly, so you need microsecond pulse durations of very, very high power. The "normal way" as you put it.