Requirements: a friend who has the map pack.

Step 1: You and your friends both start separate custom games.
Step 2: Get a lot of points but don't go past round 1. (chalk all but the church unless you can do the church)
Step 3: Roll the box multiple times and try to get the time bomb. After you get the time bomb throw it down and get your friend to join your game. (since he wasn't present during the time bomb he wont lose points when you detonate it.(he will lose guns and i think perks))
Step 4: With your friend in the game have him obtain 15,000+ points. (Use your points to open as many doors as you can and get Leroy to open all barrier)(Don't forget the power)
Step 5: After your friend thinks he has enough money detonate the time bomb.
*optional* Get some weapons from the box or wall weapons
Step 6: Go to the mansion and make your way through protecting your friend as you go, make it through the maze and PaP his weapon. then you can get him to do the same for you

Good luck!
Congratulations you now have that achievement =D