I have a silver trophy in Destruct-O-Match II, and I win World Challenges all the time. Here's my general method:
1. Get rid of the countdown block.
2. See which color contains the block that, when you click it, makes all the blocks of that color disappear (the block with the circle and arrows). Get rid of as many blocks of that color as you can manually, and when you have just a few left, click the special block (you don't get points for the ones that it vanishes).
3. In early levels, you'll now have three colors left. Pick one of the remaining colors BESIDES the one with the question mark block (assuming you have a question mark block). Try to get rid of all blocks of that color.
4. When you're down to two colors, try to group all the ones of the question-mark-block color. Click that, and a shower of blocks will fall from the sky, all in the one remaining color.
5. Click any block and they'll all disappear. If you have just a couple blocks left surrounding the fire block, that's okay too.
6. Finally, click the block with the fire on it. As (or after) your score is totalling, type "destroyboulders" for an extra 250 points (only if you have completely cleared the level. You can do this only once per game).
In later levels, you will have more colors, so you'll have to try to group them and get rid of as many as you can before hitting that question mark block. I usually try not to hit it unless I have at most three colors including it -- say, if it's blue and I also have green and orange, I try to group all the blue, then hit it, so only green and orange fall. Then I can almost always get rid of all of those easily. You want to have four blocks or less left at the end of each level for bonus points.
Generally, I try to have a score of around 650 after level one, 1000+ after level two, and 1200+ (preferably 1400+) after level three.
Good luck!