Level 25 is the first hard level in the game. To beat it, your priority is to clear some space to work with, and only then start collecting the gnomes.
Destroy the boxes at the sides of the field as soon as possible, then focus on unchaining the lower pieces. Keep track of the entire field, not just one side. Save up your power-ups and explode them together, and don’t use the Rainbow Blast too early — first, clear the field of obstacles to have enough pieces for the Rainbow Blast to collect.
After clearing the obstacles you will have a large number of auto-matches as well as power-ups, greatly accelerating the collection of gnomes. Don't forget about the gnomes hidden under the boxes in the center of the field, though they should be collected last with the auto-matches and power-ups accumulated earlier.
Be sure to keep an eye out for the tiles that are hard to access — as a rule of thumb, they are situated in the corners and on the edges of the field.