Level 54 can be tricky in a sense that it requires you to spend your moves wisely. First, clear the boxes in the middle to get access to the flowerbeds. The stream can be easily used to your advantage: plan your matches ahead so that when the tiles shift forward, more matches follow.
The flowers grown by the flowerbeds can help you in situations when you’re left with few favorable matches — it’s best to save up a lot of them, as they will often clear themselves automatically, spawning in a row after a few matches next to a flowerbed.
The remaining gnomes shouldn’t be hard to find, and it’s best to watch out for your remaining moves keeping in mind the level goals you’re likely to be short of by the end.