Q Who? Had Q and Guinan serve as their "reps". Explaining the Borg to the Feds.
Out of curiosity, what are they (leaving aside those species who have evolved beyond corporeal existence, like the Metrons, the Q, etc., species that wouldn't engage in armed combat)
In TOS they had things like the Doomsday Machine, or V'Ger, or the Whale Probe. All of those things were more powerful than the Borg.
In TNG they had things like Nagilum, or the leftover tech from the T'Kon Empire, stuff like that. All more powerful than the Borg.
What was strange was that they were so afraid of the Borg to begin with, when they already knew there were worse things out there.
Forgive me, but that's not an impressive list. We know nothing of the species that made the Doomsday Machine or the Whale Probe, and having the Doomsday Machine would not guarantee that the Borg would not assimilate the species. Moreover, we really don't have a basis for comparing how they would perform in armed combat with the Borg: the Whale Probe was no weapon, and the Doomsday Machine was able to be defeated by 23rd Century Federation technology and strategy. V'Ger? Given that it was, in part, constructed by humans, a species assimilated in an alternative timeline, it's ability are difficult to compare. At the end, none of these are the species themselves, just bits of technology. Same thing with the T'Kon: it's not clear how they could defend themselves, and their ability to resist the Borg would be entirely a matter of speculation.
Nagilum, like Q and the Metrons, really aren't a good measure either. The Borg aren't likely to be interested in them, being that they have neither physicality nor technology. I have no doubt that they could make the Borg disappear, but they aren't likely to engage the Borg. Simply put, they are unlikely to be adversaries to or a threat to the Borg, and the Borg would not try to assimilate them. We even know that Q would not judge the Borg in the way he would humanity. Those races are of no concern to the Borg. Moreover, the same could be said of the Federation's relationship with the same species. Picard et al could reasonably believe that an intellectual appeal would change their minds. The Borg offered no such dialogue, just a commitment to assimilate them. There was no room for negotiation.
Q and Guinan are "representatives" to the Borg? That's strong language.