I couldn't figure out how to get gold on 7 and 18 in 1 slice without watching the walkthrough, even after I scored over 1520% in total. (at this moment lv7 was 0% and 32% on lv18)
If you feel really stuck, take some guesses and see the numbers; they get highlighted only if correct matching cells are surrounded by X's AND all cells toward at least one side are marked with O's and X's.