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Abridged Version: Do it on the map COMPLEX. Use THERMOBARIC ROCKET and TREMOR PACK. Soften up the buildings with rockets and be sure the tremor finishes them off. Other settings don't really matter.
Some people recommend different tactics, most of whom suggest the mighty THERMOBARIC ROCKET because of its big explosions. However, it seems you get more points for letting the building collapse on itself, and that the SINGULARITY BOMB, ROCKET LAUNCHER, or NANO RIFLE are the best.
In my opinion, and I tried all of these, the thermobaric rocket is the best, and easiest to use. I scored decent points with everything else, but (this is most previlient for S.Bomb) I had trouble taking down all the buildings within a minute. The thermobaric rocket may not yield the BIGGEST scores, but you'll consistently score high.
As for the backpack, most people say it doesn't matter. However, a good amount of people suggest the TREMOR PACK. Since you score bigger for making the buildings fall down instead of go BOOM, this is true. If you weaken the supports on the building(s) and leave one or two, then finish it with a tremor, you'll get (on average) 10-20k more than you normally would.
With a combination of THERMOBARIC and TREMOR, I was consistently getting scores around the 130k range, with 150k tops. Not nessissarily as high as some of the others, but a decent score that's very easy to reach.
Lastly, even if you set it to 4 players @ 5 rounds, it counts EVERYONES score to the achievement. And if you quit out before it ends, it counts everything up to that point as well. It unlocks after the round where you surpass 25 million.