The Collaborative International Dictionary of English v.0.48:
Libeler \Li"bel*er\ (-[~e]r), n. One who libels. [Written also libeller.] " Libelers of others." --Buckminster. [1913 Webster]WordNet (r) 3.0 (2006):
libeler n 1: one who attacks the reputation of another by slander or libel [syn: defamer, maligner, slanderer, vilifier, libeler, backbiter, traducer]