You earn this achievement by scoring more points than the target score, which is shown in the upper right corner of each level. You *may* be able to get this without playing on Expert, but most levels require you to play on Expert to earn the A+ rating. Beating the first level on Expert isn't hard. My solution was to place dynamite about 1/2 way between the second vertical leg touching the ground and the vertical leg of the upper structure. A picture is here:http://www.4shared.com/photo/YjJS_zpu/P1-01X.html
or you can see my solution for other levels here:http://implode.4shared.com/
The achievement pops when the score shows up, which is when it decides that you have successfully blown up the building and determined your score.
If you have any questions or have any trouble duplicating my results, please let me know!