I confirm that you can select and finish the 50 or 25 wave (thank you Princess BenKun ), just choose any map, grab your friends, I suggest to play as the sniper and use the following cards
You and your friends commit suicide the scout will pick up your energy, and use sniper strike or team revive if one of your buddies get down.
A speedrun is the best option, play on lift or War Machine, there are a lot of sessions with those maps.
I enjoy playing as the Soldier, I use these cards
1 hour per speedrun, you will get the cheevo on 3 hours
if you like to play by yourself play the juvie madness option, just go to settings and select juvie madness
Then play as the scout with the following cards, you can change the first one for the shotgun capacity, I recommend play with cards at lvl 3 or 4 (minimum)
Juvie madness is a match with 25 waves of juvies and the trackers (the enemy ball) waves 5,10,15,20 and 25 spawn bosses (pouncers, robots, scion) you can handle it. (1 hour 45 minutes average)