A multigrid optimisation strategy is introduced to design passive metallic reflectors with corrugated shapes. The strategy is based on using genetic algorithms at multiple grids and shaping the metal sheets, starting from coarse details to fine tunings. This corresponds to a systematic expansion of the related optimisation space, which is explored more efficiently in comparison to a brute-force optimisation without using grid. By employing the multilevel fast multipole algorithm to analyse the electromagnetic problems corresponding to optimisation trials, we obtain accurately designed reflectors that provide focussing abilities with very high performances at single and multiple locations. The designed reflectors are also resistant to fabrication errors with less complex corrugations and simplified reflection mechanisms compared to those found by no-grid optimisation trials.