Level 15 needs continuous bursts

level-15-2-Stars

Starting Bubbles: 16
Recommended Potions:
Yellow Spiders
Points needed for 1, 2, 3 Star:
30 000, 60 000, 100 000
You’ll need to burst bubbles continuously to generate green spiders. Try to do the first five burst with minimum bubbles falling into the cauldrons, so you can build up spiders for the rest of the bubbles to fall. With the sixth burst, you can let lots of bubbles fall into the cauldrons, since you have a green spider.

There is a good general tip in the manual for level 14, that you can look at.

This Upper video walkthrough have the Charm of Shielded Spiders active (you notice that at 0:41 in the video) and the Magic Potion Yellow Spiders.

Here is a video walk through for level 15 brought to us by 0TheDanishCommentary. The video has a loud music track, so you might want to turn off the sound.