Played a few levels on the smaller grid sizes, the couple I tried on the larger sizes were pretty maddening. I'm not sure I'll make it through all 800... Is this np-complete?
The levels are in order of difficulty per grid size, sorted by the number of backtracks it took a solver to complete (some of the harder levels on the biggest sizes took hundreds of thousands of backtracks, though I didn't really code any heuristics).