Guera has offered excellent advice. It would be much easier to offer advice if you had provided more information on what you have tried in the past. I'd suggest slow incremental steps and remember that you don't need to strike harder to leave longer lasting marks. As an example, if you have beaten her ass with a paddle, try using a narrower implement, such as a long ruler. Striking with the same force, but on a narrower surface area will leave a longer lastimg bruise.