I know most of the solutions for this are very similar, but I'll offer my take anyway and add a few more things.
Mode: Total Chaos
Weapon: Thermobaric Rocket
Building Collapse: High
ROUNDS TO WIN: 5
Each round will take about 5 minutes, with each player I was averaging between 115K - 125K for score while firing the rocket and using the tremor backpack as frequently as possible.
So every round, this will gain you 460K - 500K give or take. It is possible to do less and more per round.
All players scores in this count towards the progress of one's gamertag.
If you're smart doing it this way, you can have each player win 4 rounds each, making the longest game at max, 17 rounds.
460K X 17 rounds will score you: 7.82M
500K X 17 rounds will score you: 8.50M
This should take roughly 1.5 hours to run through completely to achieve these scores in one game.
If you can achieve the higher scores on the spectrum, this might take you 3 games to complete the objective, if not then it should take no more than 4 games.
Time wise, you should be grinding this one for at least 4.5 - 5.5 hours to get this achievement.
Hope this helps!