Many dislike the matchmaker not taking into account the class you searched for a match as, well here is a foolproof method to play the class you want every time.

Step 1: Set the lobby to private.

Step 2: Select the mission you want to play.

Step 3: If not already the class you want to play, swap now.

Step 4: Switch lobby back to public, the lobby will now drag people searching for a game into your lobby.

Step 5: Either wait for brothers to arrive or start the mission.

Courtesy now dictates that any joining swap off whatever class you selected rather than you swap off.

Hope this helps!

This is unfortunately the best way to play the class you want online. I personally just start the game a soon as i can using the class i desire so those joining with a same class don't really have a choice but to change. Doing this reduces unfavorable encounters and having some try to force a change upon you because their level is higher. 

Honestly I am the opposite, all my classes are 25. If someone spawns the same class as me and are not 25, I swap.