There are several PR firms, lobbying groups who specialize in mud slinging, making wild allegations on behalf of their clients. Is there a word that describes such a person whose job is to make allegations against others?
one who spreads real or alleged scandal about another (usually for political advantage)
A muckraker spreads scandals, usually for political advantage. Being a muckraker is considered sleazy.
Muck can mean feces or dirt, and a rake could spread it around. Similarly, a muckraker spreads around something that's dirty in another way: news of scandals, real or fake. Muckrakers are kind of like gossips, but they're more public. Many politicians are muckrakers when they talk trash about their opponents. Newspapers can be muckrakers too, if they spread word of scandals, especially ones that are hard to prove. Muckraking means about the same as mudslinging.
I'd suggest hatchet man, as defined below by "Dictionary.com Unabridged":
a writer or speaker who specializes in defamatory attacks, as on political candidates or public officials.
a person noted for harsh, personal, and usually public verbal attacks against others [a political attack dog]
It makes me think to the famous principle "slander boldly, something always sticks" (from Francis Bacon "audacter calumniare, semper aliquid haeret").
Then, professional slanderer may fit.
"Slander" definition: defamation, calumny, malicious rumors, false and defamatory statement or report.
Another possibility is a drive-by-smear expert.
"Smear" definition: an untrue story about a person that is meant to hurt that person's reputation
Example: I think the parliamentary secretary should apologize to them and to this House for his drive-by smear.
You could consider using "smear campaigner". "Smear campaign" means:
A plan to discredit a public figure by making false accusations.
"Well, Senator Kerry had to respond to these scurrilous attacks on his character as a smear campaign."
[Oxford Online Dictionary]
"Negative campaigner" could also be considered as "negative campaigning" means:
Negative campaigning, also known more colloquially as "mudslinging", is trying to win an advantage by referring to negative aspects of an opponent rather than emphasizing one's own positive attributes or preferred policies.
As a reference to patent troll, you can always say PR troll or similar.
The authors of such campaigns are sometimes called a mud machine (see Wiktionary entry for quotations) by their detractors.