Go to menu - online - series - Overrun.
Objective: capture as much territory as you can to win. Most feature at least 3 rounds to win.
Recommended - spam you run button and equip a your free pump action shotgun to shoot anyone who gets in the way.
Time to complete: sometimes I got easy rounds with a few newbie players which helped me clear this achievement. Even if you queue up in the same lobby multiple times you can end up in a 8v8 match. When you have fewer than 4 players you can end up with many more before the match ends.
It can take anywhere from 30 minutes to 6 hours depending on your luck and skill. Just go for the objectives where you can and forget about killing folks if not necessary to win. The train mode is the most difficult to win because you constantly have to defend and experienced players have an advantage with more items, better guns, health etc. so expect to lose sometimes when to odds are not in your favour. You can still learn more about the game in the process.
Enjoy and good luck with the achievement hunt!