Level 90 leaves you with a few free tiles, which means you initially don’t have that many possible matches. However, you will still be able to move to one of the branches to the top left or right, or down to the central tiles. In case you have favorable matches in the center, you will get the space to create larger power-ups, whereas boxes on the top left and right contain a Rainbow Blast each. It’s better to activate both on the tiles most difficult to access (such as the chained ones in the central part of the field).
Afterwards, try not to get sidetracked into collecting gnomes individually: use the numerous tiles in the middle to create power-ups. If you explode several of them at the same time, you charge the Rainbow Blast faster, which will help you get the remaining gnomes.