The problem you think exists is none in reality, but instead two different situations there, that SEEM similar but are not.
The Borg are able to adapt quite fast to all weaponry. This means a specific weapon may work once, or twice but then the Borg have adapted. Thus humanity HAS already designed weapons against the Borg, but they have proven not efficient enough quite fast (as they try to out adapt beings used to adaption).
Species 8472 on the other hand has had technology that by itself was beyond the Borgs. Thus they were declared a primary target for assimilation. BUT the problem is their technology was so high that they could simply destroy Borg cubes sent to assimilate them; which they did and counterattacked.
As Valorum has mentioned in his answer: Borg lack ingenuity. Which is what humans have in abundance. Thus with our own way lower than Borg technology level, we can sometimes even the playing field against the Borg a bit until they adapt. With Species 8472 on the other hand...they had a technological advantage the Borg were unable to counter as no other species they had assimilated or could assimilate had appropriate technology. So they needed ingenuity which they got in the form of the Federation crew joining sides with them.
What is more though is that the Federation was too low in tech level to make a difference, thus the Borg changed that by helping out with their own tech. Thus the Borg Tech and human ingenuity make the difference there.
But back to the question: The problem is that the 2 situations are completely unrelated. Borg are adapters without ingenuity, while Species 8472 have a VERY high technology level, but are not as adaptive as the Borg or humanity. So aside from the Federation being the underdog with quite some improvisational talent in both situations. Those two situations are radically different from each other.