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.