It's highly likely that the Borg have assimilated species which are telepathic enough to sense the presence of a Q. That, or otherwise put up a fight through a psychic battle. We've seen some incredibly powerful beings which, while not omnipotent, could certainly be a contestant against Q in the mental realm.

Was Q assimilated by the Borg?

Could "Q" or a "Founder" be assimilated by the Borg? Q: no. Changelings (founders): not really, since borg nanoprobes and technology probably aren't compatible with non-solid species.

Who is Q afraid of in Star Trek?

Guinan later admits to Picard that she's dealt with the Continuum before, but her dislike for this Q, whom she calls an imp and a trouble-maker, goes without detail. She's also the only one to realize where Q has sent Picard's ship because the Borg destroyed her homeworld long ago.