There is no preferred map for this achievement, although Urban Chaos seems to be the most common choice among players due to the presence of Bloodshots and Zombie Dogs (which can kill you very fast). Make sure to communicate with your partner at all times.
Player 1 (who isn't going for the achievement) should let themselves lose some Health during the beginning of the match.
Player 2 (who is going for the achievement) should build up a combo of at least 15 enemies before sending them over to their opponents map.
"Sent-Over-Enemies" will have a green outline/color so it is easy to tell which enemies belong to your opponent. Player 2 should let Player 1's Zombies down them, and hopefully those same Zombies finish Player 1 off.