fairly simple, do arena with your favorite fighter who you feel comfortable with and make to the end and beat the final boss.
here's a list of each person you'll face with the fighter you choose-
Kuma (Press X when highlighting Kuma): In order, you will fight Panda, Paul, Jin and Azazel.
Baek: In order, you will fight Law, Hwoarang, Jin and Azazel.
Lei: In order, you will fight Feng, Kazuya, Jin and Azazel.
Nina: In order, you will fight Lars, Anna, Kazuya and Azazel.
Marduk: In order, you will fight Steve, King, Jin and Azazel.
Paul: In order, you will fight Leo, Kazuya, Jin and Azazel.
Raven: In order, you will fight Jack-6, Heihachi, Kazuya and Jin.
Feng: In order, you will fight Lei, Asuka, Jin and Azazel.
Kazuya: In order, you will fight Lars, Heihachi, Azazel and Devil Jin.
Jin: In order, you will fight Lars, Azazel, Kazuya and Heihachi.
Dragunov: In order, you will fight Eddy, Nina, Jin and Azazel.
Armor King: In order, you will fight Bryan, Kazuya, Jin and Azazel.
King: In order, you will fight Bob, Marduk, Jin and Azazel.
Julia: In order, you will fight Ganryu, Bruce, Jin and Azazel.
Anna: In order, you will fight Lars, Nina, Jin and Azazel.
Law: In order, you will fight Baek, Marduk, Zafina and Ganryu.
Ganryu: In order, you will fight Julia, Jack-6, Jin and Azazel.
Roger Jr.: In order, you will fight Bryan, Kazuya, Jin and Azazel.
Mokujin: In order, you will fight Leo, Zafina, Jin and Azazel.
Jack-6: In order, you will fight Alisa, Heihachi, Jin and Azazel.
Bryan: In order, you will fight Alisa, Yoshimitsu, Jin and Azazel.
Eddy: In order, you will fight Lars, Anna, Kazuya and Azazel.
Christie: In order, you will fight Eddy, Nina, Jin and Azazel.
Xiaoyu: In order, you will fight Asuka, Heihachi, Jin and Azazel.
Devil Jin: In order, you will fight Lars, Heihachi, Kazuya and Azazel.
Asuka: In order, you will fight Lei, Feng, Jin and Azazel.
Bob: In order, you will fight Ganryu, Paul, Jin and Azazel.
Leo: In order, you will fight Bruce, Anna, Kazuya and Azazel.
Miguel: In order, you will fight Eddy, Nina, Kazuya and Azazel.
Zafina: In order, you will fight Raven, Kazuya, Jin and Azazel.
Lili: In order, you will fight Alisa, Lars, Jin and Azazel.
Hwoarang: In order, you will fight Steve, Baek, Jin and Azazel.
Heihachi: In order, you will fight Jack-6, Kazuya, Jin and Azazel.
Lee: In order, you will fight Heihachi, Kazuya, Jin and Azazel.
Steve: In order, you will fight Heihachi, Nina, Jin and Azazel.
Bruce: In order, you will fight Lei, Nina, Jin and Azazel.
Yoshimitsu: In order, you will fight Alisa, Bryan, Jin and Azazel.
Wang: In order, you will fight Lars, Heihachi, Jin and Azazel.
Panda (Press A when highlighting Kuma): In order, you will fight Kuma, Heihachi, Jin and Azazel
all credit goes to the Authors of Tekken 6 walkthrough at
TrueAchievements