Once you get over the learning curve with the controls, it's fairly easy to beat. Just demands a lot of concentration. I started on 3.0 intensity at the beginning and pretty soon I cranked myself up to 7.0 for all the missions. Still can't really handle 9.0 intensity, but eh. Anyways, if you're having trouble, the game'll knock down the intensity/difficulty every time you die or you can just set it low yourself before the stage begins.
I'm fairly certain the NES game didn't have an intensity slider.