I would give you a level-by-level guide, but obviously I can't remember each level off by heart, and it hardly gives enough time to write anything during the level, so i'll do what I can

Firstly, on all levels 2-10 you should stall for time and get all the pickups the level offers before you kill all the enemies.
Secondly, collecting the CRISTAL letters in the CORRECT ORDER i.e. C then R then I etc., gives you a bonus of 150K POINTS. This is much harder to do than it sounds.
Thirdly, onto the 0-10 level guide.
Level 0-Go to the very top left hand platform, jump around a bit and a coin should appear. DO NOT blow up the well, instead jump down and hold the Left arrow down, and you will enter a secret passage.
Level 1-If you have done it right, various crystals should fall with you. Do what you can to get as many as possible, I myself can never get the red one.
Level 2- Havn't found anything worth noting
Level 3- In the top right area jump around for a coin
Level 4- Kill the CHERRY first, and then the orange. This will give a green 10k point crystal
Level 5-Kill the TOP orange first. This is harder than it looks, as a lot of the time it bashes into the other one on the way down.
Level 6- Kill the cherry within the enclosed shape first. This is best done by getting the blue bomb, jumping inside and placing a bomb on the far left of the enclosed shape. The cherry should then fall into the hole.
Level 7- Kill the tomato which starts higher up first.
8-Firstly, jump down and get the orange killed. Then, remember which tomato started in the top RIGHT, and kill this one. Leave the top LEFT tomato to kill last. If you get it right, a red crystal worth about 22k should be yours. Also, jumping around over the two ll shape blocks inside the enclosed area gives you a coin.
9- I have never gotten anything from this no matter what order i try, so can't help here sorry.
10-It's pretty easy to kill them all in one shot, however there is an order which grants you a good crystal. I've done it once but I can't remember which order they died in, sorrrrry.
So, hope this helps, and maybe i'll do a level 11-20 guide soon

One last tip, on level 16, there is a way to get a giant orange crystal worth neary 100K POINTS on itsself. I've done this ONCE before, and all I know is that the top cherry died last.