1) Backbiter 2) Backbiting sort 3) Depreciator 4) Detractor 5) Disparager 6) Knocker 7) Libelant 8) Libeler 9) Maligner 10) Mudslinger 11) Slanderer 12) Traducer 13) Vilifier 1) Backbiter 2) Libeler 3) Maligner 4) Slanderer 5) Traducer 6) Vilifier 
• (n.) One who defames; a slanderer; a detractor; a calumniator.
De·fam'er noun One who defames; a slanderer; a detractor; a calumniator. 
[n] - one who attacks the reputation of another by slander or libel
defamer A person who attacks the reputation of another person with slander or libel.
maligner noun one who attacks the reputation of another by slander or libel