Trying psi on the Borg, I would set the difficulty as if hitting the entire collective (at least a few thou' on the cube or whatever; more if they're in contact with other ships...) They don't appear to use psi, but if they've got a psi race, they might be able to use that drone -- I don;t think it would apply across the collective.
"War is an ugly thing but not the ugliest of things; the decayed and degraded state of moral and patriotic feelings which thinks that nothing is worth war is much worse. A man who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."
John Stuart Mill