After testing all the levels again, I found that level 45 will allow you to get the Chain Reaction x20 achievement. You start with 3 split bombs and 2 spike bombs, but there are chances to get another 2 split bombs and 2 helicopter bombs, a total of 21 'sub-bombs' if you play it right. (Hint: shoot the split bombs off to the side and use the spike bombs to pick up the bonus bombs.)UPDATE: with the latest batch of levels released, the x25 chain reaction achievement is now possible, via level 87. (I assume getting the x25 here will also give you the x20 but I haven't tested.).
On another note: Woodstock should change the name because apparently people think it will play Amazing Grace. It's a wind chime not a music box. It's not going to play the song Amazing Grace. It's tuned to those notes and it should play those notes randomly (I say 'should' because mine don't play anything at all).