First of all, try to remove the stones blocking the portal as fast as possible. To do it, you should create and explode larger power-ups.
Then you should make matches and explode power-ups next to the flowerbed. No need to worry about collecting the flowers individually. Some of them will be collected by auto-matches, and a Rainbow Blast can help with the remaining ones.
Each 3 matches or explosions next to the flowerbed make it produce 4 flowers on the field, so to get 16 flowers you will need to make 12 matches or explosions near the flowerbed. Thus, the more auto-matches you make next to the flowerbed, the better. No need to worry about collecting leaves; they will be easily collected by the end of the level.