You earn this achievement by scoring more points than the target score on the first 120 levels. The target score is always shown in the upper right corner of the screen. 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.
My solution for all of the levels needed to get this achievement are here: http://implode.4shared.com/
The achievement pops when the score shows up on 12-10 (or the last level you play in the 12th zone), 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!
Also, you can get these by skipping a zone. For instance, you could get A+s on all levels in zones 1-6 (skip 7) and 8-13.