Hard Killer Sudoku starts with only a handful of given digits. From there the cages do the work: list the possible combinations for each sum, cross-check them against neighbouring cages, and use innies and outies across box borders to pin down single cells.
Expect a solve that leans on note mode from the first minute. Every hard grid is verified to have one solution, so patient arithmetic always gets there. Refresh the cage techniques in our article on Killer Sudoku strategies.