When your stamina drops, block and recover. Also do this when your opponent goes on the all out on offense.
Punching
Do not randomly punch repeatedly without any thought to what your opponent is doing. This will result in a waste of stamina, which will slow your punches and force more lag at the end of your move, which leaves you open to damage. Instead, be moderate when punching and conserve stamina. When it gets low, switch to defense. Try to land as many hits as possible. Each of the three star ranks are based on how many of your punches actually land. Blocked attacks will not count towards this total. Charge your jabs, hooks, and uppercuts, by holding instead of tapping. Your gloves will begin to glow to indicate that your punch is fully charged. When released, your character will unleash a stronger punch. They are more efficient than quick light hits. However, when your opponent is blocking frequently and you cannot land a charged punch, consider to use occasional fast punches instead. When your opponent is using charged punches, make sure to block them.
Training
Use the gym to supplement your training. While the timer is running, each action in the gym will help upgrade that particular stat point. After enough points are accumulated to fill the bar, you will increase that stat permanently by one point.