Well, basically...

She is a magical girl using a device called Raising Heart. Nanoha is kind of a magitech magical girl series, and her fighting style is flying around and firing off blasts of magic ranging in size depending on how powerful the spell she's using is, and using an outfit called a barrier jacket that protects her from many forms of harm(guns, for example), though it's not impervious to everything or anything like that. She has a tendency to do overkill and use firepower as her response to most situations, but none of her attacks are at all lethal. They can be set to that, but she'd never do so, she just knocks her enemies out.