@SeraphicStar some levels are tricky and you need to find right strategy (but it is rare). Mostly you relay on probability, just keep trying. I don't remember the level number I was grinding it for couple days!!!
Another strategy is to maximize usage of 2h bonuses. Say, if you need move hearts and you have 2h fish bonus (sorry, I do not know the names) you may want to use Owl (pink) to generate more bonus fishes (every 8 candies instead of every 10 for red).
Nice suggestion about the fish and character choice @k55. I never thought about that but will definitely keep in mind for next time I get a timed booster😊