To begin you will need Tatooine, Jakku, Kashyyyk and Hoth unlocked.
For this we will be using the Panic Jedi Mind Trick
. This can only be done on certain characters. To get a large number of those characters in the Cantina we will need to complete some side missions.
Go into the Cantina and activate the side mission No Arm in Asking
from Doctor Evazan. This mission is get the spare arm for Ponda Baba. You will travel to Kashyyyk to retrieve the arm parts which will also activate the side mission Armed and Ready
. Once that is complete you'll need a surgeon to complete the original side mission. This is done by going to Hoth: Echo Base and finding 2-1B who will give you the side mission Party People
. Upon completing that you'll unlock 2-1B, go back to the Cantina and talk to Doctor Evazan thus completing No Arm in Asking
. You can now do Baba Bar Brawl
from Ponda Baba, which will add all the characters into Cantina required for the achievement. I completed the side mission first and remained in the room to do the achievement.