Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to correct your manager's factual mistake, use this factual base: use recent examples, observable impact, and the work requirement rather than labels about personality.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask for a specific change, decision, or working agreement and choose a time to review it. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not diagnose, gossip, embarrass a colleague, or turn one incident into a judgment about their character. Move harassment, discrimination, threats, health risk, and retaliation into the appropriate protected process. You do not need the other person to agree with a boundary for you to follow it consistently.