This solution will get 2 people the achievement per 30 minute match (you will be playing the full 30 minutes)
To start, you need 6 people for a ranked match. Start a ranked Devastation match on Trenches, with the win being for 5 targets for 30 minutes.
The bomb is on the roof of the building in the center of the map (there is a ramp on each team's side to get up). Decide which 2 people will be getting the achievement. One of them will take the bomb, and take it to the nearest target. There are 2 targets in the center area. Facing from any team's side, the target is the closest building on your right. The other target will be diagonally across the center area from there.
Once the person has planted the bomb -- done simply by running over the target -- the other player getting the achievement will just run over the target (sometimes you need to wiggle around to have it read you're there) and it will say you've disarmed the bomb.
This doesn't return the bomb to the start location, it gives the bomb to the disarmer. Sprint across the center area to the other team's target (the team that planted the first bomb) and plant it there. The person who originally planted the bomb will now disarm it.
Rinse and repeat running back and forth, planting and disarming, and you should each hit 50 disarms just short of the match ending -- maybe 2 minutes left, I usually had about 1.