In order to target the subcutaneous fat stored in your under arm region, aim to do 30-40 minutes of intense cardio 3-5 times a week for best results. This will get rid of the visible layer of fat and help to tone the muscles.at Pull Downs. Lat pull downs are conventionally done for toning the back muscles but they are also a great way of toning underarm muscles, as this is the muscle you are using while you bring the bar down. Do 2-3 sets of lat pull downs with 8-12 reps for best results. As you get stronger, increase the weights.