Low weight, high reps and a lean diet may be the way to go.
Ask any serious body builder and they will tell you that the key to a lean body is not cardio but it is having a good diet. As long as you take enough protein each day, which is 1-1.5 grams of protein for every kg of body weight, spread this throughout the day split within 5 meals, every 2 hours or more, less meals does work also as fasting routines can show great results, have a small amount of starchy carbohydrates with each meal so your body produces insulin to carry the protein to your cells and some green veg or other nutritional veg for the vitamins and antioxidants with one meal at least will keep your body healthy.
Carbs are the main enemy when trying to loose weight but we still need small amounts to carry the proteins to the cells and provide energy for the day and training. Many people use cardio to keep lean but this can be dangerous when taken to extremes, so moderate cardio is fine and a good idea but mostly for stamina and lung capacity but the problem with doing cardio and trying to build mass with weight lifting is the cardio interferes with the amount of muscle your body is able to build. 860
Most body builders recommend 25 minutes of light cardio, 3 times a week, preferably on off days, doing it before or after a weights session will impact the energy you will have left to train and the recovery of your muscles.
There is a debate over light vs heavy weights in the body building community, both provide good results but the problem with training with light weights is that you need to do high reps, so 15-30 and over time this puts a strain on your joints and can wear them out, but light weight high reps does work, if you look at rowers and swimmers they have great shoulders and back, and just using a rowing machine can build good definition and mass.
It also depends on if this superpowered persons muscles reacts to stimulus in the same way as a normal person if so then even heavy weights wont seem super, I have trained with people who can lift 300-400 kg on most compound lifts and I personally can lift 340 kg on a bar for some exercises not included machines which you can lift 400 kg over on, so anything under 600 kg is not super strength, it is just normal person strong. But if this super powered beings muscles cannot be stimulated by normal peoples weights then they wont be able to build mass, it would be like us trying to get big with 1 kg dumbbells. In that case some secret truck lifting at night will be needed. 435