I am using a monster named Meemo, who has all 3 Stray Angels as boosts. Most people think its about forming larger chains but the real score comes from "clearing" the board. The Stray Angel boost makes this easier.
Another tip. If you can't find chains anymore, double click on the board so the chains you formed will be removed and the next gems will drop. This will save you more time.
Trust me, I made it through the Coast without the need to repeat a stage as I all got three stars. Then I went back to Cave and Tundra to finish the rest. Top score is 3M, and I am only using a mouse.