Word for a person who intentionally maligns someone's image in front of others

I have faced such situation twice or thrice in my university and work life. Due to competition your peer or ill-wisher tries to put you in dark light intentionally. So, it gives wrong impression of you to other people. If your ill-wisher succeeds, then your image is spoiled in front of others without you actually doing any wrong.

So, what do you call a person who spoils your image or malign your image in public?

For example: He is a/ an ____. He spoiled my image in my office.

In real, he did not spoil my image, but he tried and my co-workers supported me. One more thing, I use to use ill-wisher until now but I feel I need one perfect word. Using ill-wisher sounds stupid.

The richness of English for approximate synonyms suggests that you are far from being alone in having been upset by a:

vilifier, traducer, disparager, denigrator, panner, defamer, knocker, and maligner!

Most of those synonyms imply an element of untruth (libel, slander) so they may not quite express the meaning you seek.

Slanderer (noun): a person who slanders someone (= damages their reputation by making a false spoken statement about them). (Cambridge English dictionary)

Examples:

• I described him as a serial slanderer who poisoned political debate.
• The documents prove that they are liars and deliberate slanderers of my name.

(There's nothing wrong with 'ill-wisher'. You could also call him 'antagonist')