The easiest places to get massive D are the museum and Zakarov's Penthouse. In the penthouse, the mines will do a lot of damage for you, while in the museum there is lots of stuff to break...exhibits, showcases etc. Keep giving it all you got and you'll soon exceed 75.000.000, which might require more than one playthrough. Make sure you also completely vaporize the dinosaur skeletons in the museum to get the Paleontologist achievement as well.