Load up a private match on any map: Select one of the following characters: (The number inside the brackets is the operation the characters are from). The achievement description says characters from operation 2 or 3, but operation 1 seems to be working as well...
Operation 1 and 2:
Baird,(2)
Lizzie Carmine,(2)
Drone,(2)
Jermad,(2)
Raam,(1)
Warden,(1)
Kantus,(2)
Cog Gear,(1)
DeeBee,(1)
Operation 3:
Cole,(3)
Clayton Carmine,(3)
Myrrah,(3)
Theron Guard,(3)
Finish the match and BOOM! achievement unlocked.