All the possible rules I come up with either leaves it extremely easy without using much of the board, or doesn't make anything I would consider a soultion... :(
Not sure what you're stuck on, but I think this might help: (rot13) Lbh vagrenpg jvgu gur chmmyr ol qenjvat n oyhr naq benatr yvar, fgnegvat ba gur znexf ng gur obeqre
