Start with properly assessing the field and don’t rush with your first moves. Given few free tiles, it’s best to account for every favorable match available from the start and use the ones in the top first. You can switch to the lower matches right after that and start clearing the way down and to the middle, destroying the chains and boxes. It’s better to go for the chained pieces in the middle, though sometimes you’ll have more favorable matches around the boxes to the bottom, so it’s better to adjust your strategy according to the available moves.
Try to combine your explosions so that you charge the Rainbow Blast faster. It can be used on the pieces located on the tiles most difficult to reach.