*Note: despite what the achievement description says, you do not need 5 gamertags on your team, just 5 in the match / room / lobby. I got it while I was on a team of 2 as I will describe below.
Although I actually got 1 out of 10 of these in an actual public match (much to my surprise, I guarantee you), the majority came from a boosting session. A really good method we used was in Team Mission. Have a team of 2 against everyone else in the session. One person can plant 3 demo charges each round (why not get that "Demo Expert" achievement, too?) while the other goes to the enemy spawn and kills them. We had respawns at 5 individual, so killing the entire enemy team didn't end the match right away. Once the first person has planted their 3 demo charges, both of you run to the extraction point and the match ends. Rinse and repeat. Doesn't take very long at all. As a bonus, you'll also get the "Team MVP" achievement the first time you do this if you don't already have it.