...But how to create an explosion bigger than the ones the Borg have shrugged off so far? The main deflector of Starfleet's biggest and baddest ship, presumably her most powerful weapon even in theory, failed to make a dent.
Anyway, if technology overwhelms the Borg, they can always adapt in terms of tactics. A weapon strong enough to annihilate Cubes with one shot? Just send a thousand Cubes! Or fire first.
Really, there's no future to devising stronger and stronger weapons, as this only makes the Borg
stronger. What one needs is a weapon that kills with one shot, yes - but also a means of ensuring that the Borg gain no information from this kill. Basically, a whopping big death ray that also emits a jamming field (say, Nero's drill ramped up thousandfold) to cut off communications with the rest of the Collective should offer a chance of success.