Over time I've become increasingly impressed with the broad applicability of Matthew 18:15-17:
If your brother or sister sins, go and point out their fault, just between the two of you. If they listen to you, you have won them over. But if they will not listen, take one or two others along, so that ‘every matter may be established by the testimony of two or three witnesses.’ If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector.
The first step is often difficult but crucial. The path of least resistance can be to go behind your antagonist's back --- to your friends, or their friends, or their manager. I've seen all kinds of negative consequences from following that path --- hurt, distrust, unnecessary escalation, confusion and fear. I feel my integrity depends on people knowing that whatever I say about them to others, they will not be surprised by because they've already heard it from me.
This applies in the other direction too, when people complain about third parties to me. If the third party is unaware of the issue, I don't want to know --- go away and talk to them first.
There are rare exceptions, usually involving time-critical emergencies or complex secrecy requirements.