Come up with a bunch of NEW weapons that can crush them before they adapt to them.
...Which is the opposite of the idea I was commenting on and declaring unworkable, that of creating a weapon so powerful that it renders all adaptation meaningless.
The 8472 had something of that caliber: a death ray capable of destroying planets. The Xindi had the same, thanks to future help. But those weapons demonstrated no anti-ship capabilities, so a force of Cubes still stands invincible against brute force attempts on screen.
First Contact, for example, would have resulted in victory for the Borg if they had simply deployed two or more cubes to the job.
It did result in Borg victory: Earth was
assimilated. This was done with subtle Borg trickery (time travel) rather than with brute force.
That it was later (earlier!) undone does not disqualify the original concept as such.
a rather inexplicable addition to their capabilities that appears to be a misunderstanding of Guinan's line
Agreed - but it's a rather exciting and facilitating addition. Trek is full of such misunderstandings (Data speaks formally -> Data can't contract was just silly; Ferengi are greedy -> Ferengi can't and won't fight was equally silly but in a good way), and usually manages to wring something interesting out of them.
"Q Who?" already showed the Borg adapting anyway: originally, Worf's phasers worked on a Drone, then they no longer did. It was part of the concept in any case, regardless of later misunderstandings, oversimplifications and plain concept evolution.