The main goal is to clear the path to the flowerbeds as fast as possible. Start with the middle part of the field to get some space to work with and then try to make larger matches: power-ups will let you hit two or more flowerbeds at the same time.
Each 3 matches or explosions next to the flowerbed make it produce 4 flowers on the field. However, it’s best to let the flowers fill up the field first, and then collect them with the Rainbow Blast. There is no need to focus on collecting drops, you will easily collect them while aiming at flowers.