^ Then you're still killing them one at a time, which will never work. I repeat: the advantage of the Borg isn't their ability to adapt, it's their ability to continue attacking -- possibly even DEFEAT you -- no matter how much damage you do to them. With anyone else, there's a possibility for "critical damage" where you can knock out a few major systems that'll render the entire system either inoperable or at least unable to fight back. The Borg don't have this problem: you can blow up their entire ship, and the two largest chunks of its debris will still chase you around, shooting at you. If you get lucky and reduce the entire vessel to scrap, the DRONES can still salvage parts from the debris and build another one from scratch. Overall, if you somehow manage to overwhelm a Borg cube and destroy it, you have to COMPLETELY destroy it, or else the last remaining pieces of it will regenerate and two weeks later it's coming after you again even stronger than before.
Adding to this point, the Borg Collective is a logistically massive entity.
As such, if the Collective want you assimilated its just a matter of resources and time. They send one cube, you waste it.
They send 10 cubes, you miraculously waste them all with version 2.0 of your doomsday weapon. They all die,but before you have time to build a commemorative statue the Collective sends 50 cubes to your doorstep-and all of them have adapted to your technology and defenses.