By breaking down their dirty deeds into a series of seemingly very minor incidents. Each one, reported separately, would appear too trivial to mention. A target who spoke of them would be seen as a problem personality, or a chronic complainer. They'd probably be told they were too sensitive, and that they need to develop a thicker skin. That's because normal people can't conceive of someone being so manipulative as to plan a series of tiny attacks, with the ultimate goal of destroying someone else.
To complicate the matter, a narcopath may also recruit one or more flying monkeys to rub a little salt in the wounds. These people may or may not know how much they hurt the target. They may be completely unaware there is a full-fledged hate campaign going on.
With narcissistic abuse, it's not each small incident that becomes a problem. It's the sum total of countless acts of aggression that are so difficult to live with.
When a malignant personality becomes busy at work, she has ample opportunity over a 40-hour time span every week to inflict pain upon her victim, blow by tiny blow. The advent of email and social media makes her job much easier, and increases the number of opportunities she has to harass someone, or to turn the task over to one of her flying monkeys.