You have two options. Both of them are easy.
Option 1: Host a Game. Start a game in Dread or Nightmare difficulty or change your difficulty to Dread or Nightmare. Use Settings -> Multiplayer and make sure the "Host Multiplayer Game" toggle is on. Open your radio with
and choose Multiplayer -> "Call for Help." When someone joins you, the achievement will unlock.
Option 2: Join a Game. I don't know whether it's necessary to be in your own Dread or Nightmare game, but you might as well be. Open your radio with
and choose Multiplayer -> "Volunteer." Matchmaking will find a game for you. Look for a notification in the upper left of your HUD. When you see it, open the radio again and choose "Join Game." If the game is set to a different difficulty than the one you're playing, you'll get another prompt telling you that, so you can cancel if it's a Standard game. You'll have to Volunteer again, and it might try to match you with the same game. Either keep retrying until you get the chance to join a Dread or Nightmare game, or try Option 1. If you join, you will unlock the achievement before the loading screen is over. You can cancel the game while it's loading if all you wanted was the achievement.