Suguru

Hard suguru is a 10×10 grid with only about 25% of cells given. The cage network is denser, and straightforward singles won't carry you through.

Cross-cage deduction becomes essential. Where cages share boundaries, placing a digit in one cage eliminates it from adjacent cells in other cages. These eliminations cascade: one placement can trigger forced moves three or four cages away.

Maintain full candidate lists. When you eliminate a candidate, check the affected cell's cage for new hidden singles. Systematic re-checking is what separates solvers who progress from those who stall.