A couple of day before a live UFC fight, go into the Live Events section and select the available fight card. You will see some of the actual fights at are happening during that UFC event. Choose any of the fights, select a winner, method they will win by (KO/TKO, SUB or DEC) and the round that they will win in.
After you've make your fight pick, you can select the match-up again and select the Play Bonus option. This will take you into the fight and you will play as the fighter you choose to win. To unlock this achievement, you need to beat your opponent by the method you chose and the round you chose it in.
In the Hunt vs Mir fight, I originally chose Mir/TKO/Round 3 but I didn't want to fight that long. So I changed it to TKO/Round 1 and just rest if I didn't get the TKO in the first round. After you successfully complete the Play Bonus, you can change it back to your original pick but you will lose the Play Bonus you just complete.