You will unlock Larry Louche after beating both Sonny Sky (Level 3) and Rachel (Level 7), Lily Sun after beating Mel Mathews (Level 12) and Rai Singh (Level 16), and Daisy Deuce after beating Johnny Spade (Level 22) and Marshall Star (Level 26). You only need to beat one for the achievement. You do not have to reach the level listed with each "boss" character, just beat the two Pros before him/her.
Larry Louche has a bully style. This means that he will be raise the bets trying to get you to fold. Just don't match his high bets until you get a good hand then try to use his aggressiveness against him.
Note: There have been reports of this achievement not unlocking on the XBLA version if you have already unlocked it on WP7. I would advise if you have both to do the XBLA version first.