A couple of tips on getting Chain Reactor:
1 - Take the extra 30 seconds offered for watching an ad. You won't need it to finish the level, but it'll count towards some extra points at the end of the level (for time left over) and help earn the score-based achievement related to this level too.
2 - Familiarise yourself with the puzzles - the puzzles in this level aren't hard (especially the first 3, very easy) but they are tricky in that if you don't pay attention you can very easily place a piece in the wrong spot. Get used to the puzzle pieces and the pictures themselves. Do a run-through or two.
3 - Take your time - maybe the most important tip. I found the best way to ensure I wasn't going to make a mistake and put a piece in the wrong spot (thus breaking the accuracy chain) was to hold onto a piece, drag it over to the puzzle and just keep a hold of it, seeing where the piece is likely to fit.
Look not only at the picture but also at the shape of the piece to ensure they both match before letting go of the piece and allowing it to fall into the puzzle.
4 - The last two puzzles - The second last puzzle (a freeway junction/intersection) and the last one (some masquerade ball masks) are challenging due to the image portrayed. I can't emphasise enough to take your time on these and, if unsure about where a piece goes, hover it over any spot where it looks like it might fit.
Remember - as long as you have "hold" of the piece (have it hovering over the puzzle by keeping in contact with it with your finger if using a tablet) your accuracy chain will not disappear. So there's no rush - take your time and ensure you get the piece in the right spot.
As I said above - look at both the overall image as well as the shape of individual pieces to ensure they match up with the piece you have "in hand" before dropping it into the puzzle. If in doubt, don't drop the piece, but double and triple check.