Since this game was free now and I just unlocked this achievement, I think I should add my way of doing it too. As already mentioned in other solutions, you can set the game to 4 players with 5 needed wins, but in the end, it doesn't change anything (except you having less loading times).
Of course, most settings are pretty generic:
Mode: Total Chaos
Map: Complex (You could also pick Vista if you want to mix things up, it can yield the same scores but it's more difficult to achieve them and relies more on your spawn)
Weapon: Thermobaric Rocket
Building Collapse: High
Buildings give more points if they collapse, Barrels destroy more parts of the building, i.e. less points. Thus, if you want the tl;dr variant: Make sure to keep as much of a building intact but make them unstable enough so they'll collapse on its own at some point.
What you want to do is to shoot a Thermobaric Rocket at the corner of the buildings and then simply keep pumping rockets straight through the hole you just opened until the rockets hit the opposite corner (you start shooting the lower left corner, you shoot rockets from that hole into the building until you destroy the upper right corner and everything that was in the way, for example) and then move on to the next building to repeat the same. Try to keep the rockets at ground level. Usually destroying most of the "indestines" and two corners of the building is enough to make them collapse with the setting at high. In case the building still stands after you let it be for a bit, pump more rockets into it or finish it off with Tremor (it doesn't really matter, just watch out that you don't get hit by the collapsing ceiling). Also, don't forget to pump a few rockets into the pipe behind the largest building and finish it with more rockets/Tremor. Collapsing buildings during the 10 second break still give points too.
I rarely dropped below 140k points/character, with my highest score being 170k. This would accumulate to a minimum of 560k each round if you play with four characters. You'd score 9.52m in a 17 round game this way, while having a pretty much failsafe method.
The achievement fill pop immediately after you finish the one-minute rampage which bumps you over the desired 25 million.
If you have any questions or negative feedback, just ask/give me said feedback in the comments!