Diet is the most important, if your calories aren't under maintenance you ain't gonna lose shit. Second, high intensity cardio isn't bad, you aren't necessarily gonna lose muscle, especially if you do it on an empty stomach or post workout (you are glycogen depleted then). Your body burns carbs first, fat, then protein, its the same reason why ketogenic diets work. The bottom line is that at the end of the day its all about cals, if you burn more than you consume than you are gonna lose bodyfat.