I agree with Vinyl, any weight training will increase strength to an extent. First, I feel that 12 reps is not very endurance like seeing as how you throw a lot more than 12 punches most likely in your classes. I think the best way to train for that kind of endurance would be running and other activities such as that, and actually doing the skill. For an increase in strength to get more power, I would say go with around 4-6 reps.