You need at least 4 to start a game. If you don't want randoms interrupting you'll need a full lobby (6 for Adam Grab).
We played Adam Grab and nobody picked up the Little Sister. We waited the full 10 minutes and then everyone got a win because of the tie. Cycle lobbies until all DLC maps have been played.
Dionysus Park
Fighting McDonagh's
Fontaine Fisheries
Pauper's Drop
Siren Alley
Smuggler's Hideout
If you don't get it right away, play another match on a DLC map (something quick and easy) and win solo, that should trigger it.
You don't have to get all maps in one session.
If you crash after a win and before you get to the next lobby you'll have to replay that map.