In the Nar Shadaa Entertainment Sector there is a Pazaak den to the right. After the guard asks you for the password go back and talk to little bat-looking creature next to the entrance and he will give it to you. Once inside you will need to defeat three Pazaak players to face the champ. They are: S4-C8, Geredi, and Dahnis. You can either beat them legitimately or each one has an alternative way to "win".
S4-C8: Ask her why she plays and if your repair or computer use skill is very high (around 15+ I believe) you can diagnose and fix her. Alternatively, if Bao-Dur is in your party offer to let him fix her and then *don't* pick the awareness option next.
Geredi: there is a group of thugs standing next to the entrance who want you to convince Geredi to come out and "talk" to them. You can use persuade if your skill is high enough or force persuade and he will go out. Note that you will get dark side points for this.
If you are a male character Dahnis will have a special dialog option that lets you win automatically. Otherwise, bring Atton and he will offer to help here.
Once you have defeated the three players attempt to leave and Champ will make a dramatic entrance. Here you can either play him which is very difficult as he has a number of very good cards or talk to him and convince him to lose with either a high enough persuade skill or force persuade to also net the Cheater achievement. Either way, you will then unlock the achievement,