you need a power puncher 2000, like the one I've shown.

People think weight training is bad. Wrong. The thing is you must do the weight reps as closely as possible to a punch. (IMO). Working max strength is fine, even better if you keep your movement explosive (enough) to target fast twitch muscles. But all slower movements designed to increse brute strength are fine so long as they are offset by bag work, because slow twitch motor units will be recruited when doing fast movements. Thus, brute strength can be converted to speed strength (power) by doing the motion more. For example, if a guy could bench 600 lb but never punched, he'd be slow. But you take the same guy who benches 600 lb but you make him train in boxing just as often, the brute strength of that bench press will be capable of delivering fast blows. Case in point, UFC fighter Tank Abbott.

If you only ever did bag work, your power would go up but slowly. If you only ever did bench presses, you'd bench a lot but your punching power would be poor. Combine the two routines and you'll hit like a freight train.

This little contraption I made has given my left arm twice the punching power. I work with is 25lb, but that's plenty.