Well, rule number 1:
Focus on the behavior, not the person.
And also, rule number 1:
Be nice and constructive about it.
If you're bothered by the behavior of one specific user, then flag for moderator attention whatever that user has posted which bothers you, describe why that bothers you, and someone in the moderation team will look at it and take appropriate action. Mind you, appropriate action does not necessarily mean the action you would take if the decision was up to you.
If there is some behavior that bothers you, then:
- Find some examples. Ideally, these should be examples by different users. If you really can't find examples of the same behavior by different users, then likely you're bothered by the user, not a behavior, so see above.
- Write up a clear Meta post that explains what the behavior is that you're bothered by, and asks how it should be handled without presupposing any particular response. Write a self-answer (not in the question itself) describing what you would like to see instead. Don't include a lot of extraneous material not directly related to the behavior you're discussing. Be constructive. Don't call out specific users, but do link to examples. Absolutely don't use expletives, bigotry or similar. You might think that following this advice somehow makes your post weaker, but it's really the opposite; showing the ability to discuss a behavior without discussing the person indicates maturity. If you don't understand why that is, then I really recommend figuring it out before you vent your frustration. It will be virtually guaranteed to be better received for it.
- Accept that the community might not feel the way you do. If it was a big problem, people would likely be flagging it like crazy, and the moderation team would be on top of it. That this is not the case likely indicates that the community is either passively accepting of or actively encouraging the behavior in question. (Now, this doesn't necessarily mean that it is right. It does, however, raise the bar for criticism.)
- Accept that moderator flags are not super votes. Moderators aren't going to go around deleting posts that some particular user just happens to disagree with. Even if the moderator agrees with you, they might feel that the type of flag you raised is inappropriate for the content in question; this sometimes happens with various kinds of "rude or abusive" flags, where the moderator handling the flag might agree that the content is unnecessary, but not with that it is rude or abusive. In such a situation, it's perfectly appropriate for the moderator to decline the flag but still fix the content in whichever way they feel is appropriate. (I myself have been pinged into private chat by a moderator to elaborate on a flag I raised, though not on Worldbuilding.)
- Accept that there are things the specifics of which moderators cannot talk about except with the one user involved, and in some cases, not even then. In addition to the usual terms of service, moderators are bound by the moderator agreement. It's short and sweet, and essentially comes down to: if an ordinary (even high-reputation) user can't see something, then a moderator can't talk about it, even to moderators on other sites.
And of course, ultimately, "appropriate action" on the part of a moderator can mean anything from dismissing a flag as invalid, all the way up to destroying the user's account on the site, but it is the moderator's decision — and responsibility — which of the available options to choose. It's the moderator, not the user who raised the initial flag prompting action, who might have to answer to Stack Exchange community managers for why they did what they did; possibly with already having had their diamond moderator access revoked, either until the issue is sorted out, or permanently.