To those who wonder about the 8x8 2-4 levels: For some of them (and other puzzles, too, sometimes), one can logically deduce that the puzzle contains more than one solution after having investigated every part of the board, and from there solving it means arbitrarily choosing a solution. The simplest case would be when all the undetermined circles form a "box" in the grid whose corners are each a 4, in which case the two solutions are 1. remove a 4 from the top-left and bottom-right corners, and 2. remove a 4 from the bottom-left and top-right corners. Either solution works (no matter what the sums are). It does not require guessing, even though it might seem like it.
For an infinitely hard challenge, search "Cruelty" in server arenas and pick the one with my name. It has 14 tanks, each are very small and fast, pack huge firepower, have loads of HP, and drop only one bubble per kill, meaning you need to able to dodge EVERYTHING. Good luck, you'll really need it!
 "Mine of Sight"){kind=icon}
 "Rullo"){kind=icon}
 "Bubble Tanks Arenas"){kind=icon}
 "Geosketch Two"){kind=icon}