Sit down on your punches. Use your legs and core to generate punching power. Slightly squat, drop your buttocks down, bend your legs while throwing power punches. Make sure you turn your waist and pivot your foot. Using your legs will make your punches more effective.
Muscle memory is what your body depends on when your brain is fatigue. Practice skills and technique with out flaw, and with lots of repetitions. So when your to tired to think in the fight, you don’t have to worry. You will go on auto pilot and your body will respond properly.
Invest in body punches. They may not seem to have and effect on your opponent early in the fight, but just keep delivering body shots and watch your opponents punches lose power, movements slow down and fatigue set in. They will fall apart right before your eyes.
The right hand lead, left hook is a great weapon against and unconventional fighter (south paw) Position your front foot to the left and outside of your opponents front foot to avoid their power hand and make it easier to defend against their jab. Once you are at this angle you can deliver a powerful right hand, left hook combo then circle to the left to get out the way.