Funzzle's hard Sudoku starts with 26 to 30 numbers and needs subset techniques: naked pairs, hidden pairs, claiming and sometimes triples. These let you clear candidates from whole rows, columns and boxes when no single is in sight. A new hard grid comes out every day, solvable by logic alone.
Naked pairs (two cells in a unit that can only hold the same two digits), hidden pairs (two digits that can only go in the same two cells), claiming (a row's only places for a digit lie in one box) and occasionally triples. Funzzle's hard grids never need X-Wings or guessing.
Two cells in the same row, column or box whose only candidates are the same two digits. Those two digits must go in those two cells, so you can remove them from every other cell of that unit.
Fill in all candidates, then look in each unit for two cells with the same pair of candidates. If there's none, look for a digit that appears only twice in a unit. The coach's first hint names the technique and the area; the next one shows the cells.
On Thursday and Friday. This page has a hard grid every day.