1. The upper blue block mustn't be moved to the left or right, as this would make the level impossible.
2. You need to get a purple block into one of the upper blue force fields to do a triple match, this will only work with the middle purple block.
3. As you can't move the upper blue block to the left or right, you have to use one of the remaining blue blocks to help the purple one reach its desired position.
4. Doing so might get you stuck, as the aiding blue block can't return to match with another blue block. Because of this, use another purple block to let the blue one go back.