DJMAX RESPECT V

While you do need to be in an Open Match for this, you only need at least 1 other person in the lobby with you. You're the only one who needs to get a Max Combo for this achievement, so you don't need to worry about the other person(s) being really good (or even playing along at all). The other player also does not need to own the DLC, so they can just be on "Observe" and the achievement will still trigger for the player getting the Max Combo.