You earn this achievement by causing 10 buildings to fall below the target level. It doesn't matter what letter grade you get. You can get this by starting on an easy difficulty (Casual, for instance), but beating 10 levels on Expert isn't hard, either. If you're going for a full 200 GS on this game, you should get this on the way to most other achievements.
Check out my A+ expert solution for 1-1 through 1-10 here: http://implode.4shared.com/
The pictures are named for the project and difficulty. P1-01X means "Project 1-1 on eXpert."
The achievement pops when the score shows up after you knock down the 10th building, which is when it decides that you have successfully blown up the building.
If you have any questions or have any trouble duplicating my results, please let me know!