Warface

Here is a quick way to do this...

Create a Plant the Bomb Versus lobby with auto team balance off.
Invite at least 5 other people into your lobby.
The people who need the achievement needs to switch to Blackwood. However teams need to be somewhat balanced in order to start the match.
Start the game up.

When in game, Blackwood guys will take turns grabbing the bomb and score 40 pts. You do not need to plant it.
In the mean time, the Warface guys cook their grenades to suicide. Press left on the d-pad to select your frag, them hold down RB until...BOOM!! The round ends. Repeat until all rounds are over. Achievement unlocked!
If your Warface buddies need it just switch and repeat.

Also note that if you have multiple people on the Blackwood team that need it and they all take turns grabbing one bomb, they all end up getting 40 pts, tying for first, and unlocking the achievement.

Here's a couple methods to net you this achievement:

Boosting:
Create a private lobby and invite your friends, and just be sure that you're in first place by the end of your match.

Versus:
While playing, be sure to use your class, like medic and engineer are probably the easiest to do this with, because you can still get a decent amount of kills, but rember to heal, revive with the medic or repair people's armor with the engineer. If you do decide to try with the rifleman, you'll want to slay and replenish other players ammo.

I hope this helps, thank you!