The word "downtoner" is a fairly new addition to the English language, having only been recognized by dictionaries in recent years. It refers to someone or something that diminishes the importance or significance of another person or thing. If you're looking for synonyms, there are several options that would work. For example, you could use the word "minimizer" to describe someone who downplays the value of another's achievements or contributions. Similarly, the word "understater" could be used to describe someone who plays down the gravity of a situation or downplays the significance of an event. Other options might include "devaluer," "belittler," or "diminisher".