Kanye West -- Pummeled 18-Year-Old Like A Boxer

Kanye West was like a boxer going in for the kill ... pummeling an 18-year old guy with more than 30 blows ... this according to an eyewitness.

We broke the story ... an 18-year-old allegedly called Kim a "n****r lover" on his way into the office of a Beverly Hills chiropractor. Minutes later, Kim and Kanye stormed in -- Kim ID the kid -- and her baby daddy went nuts.

The eyewitness tells TMZ ... the 18-year-old was sitting in a chair as Kanye made a beeline for him and, without saying a word, started slugging him in the face. The kid was covering his face but Kanye was unrelenting. Kim just stood there and silently watched the beating.

The receptionist was screaming, "Stop, stop," but Kanye ignored her. The kid quickly curled up into a ball to block the blows.