So the secret to winning a Performance of the Night Bonus, from my experience, was to finish your opponent in the first round in under a minute.
My suggestion would be to play on Easy, choose whatever weight class you feel comfortable in (I chose Welterweight), and go to the Boxing Gym and train to increase your power and strike speed. Strike stamina can help too.
Also when you're doing your fight camp, learn some perks to increase your base damage for your punches.
If you feel you're not doing well to get this, choose a kickboxing gym for your next fight camp and add some head kicks to your arsenal of moves.
Let me know if there's more to be added to this.
Hope this helps!