Keyshia Cole -- She's a Finger-Bashing B**** ... Claims Alleged Victim

Keyshia Cole knows how to brawl -- punching, scratching and pulling hair like a champ -- according to the woman who claims Cole attacked her last month during a jealous rage.

Sabrina Mercadel filed court docs accusing Cole of punching her in the face, slashing her with fingernails and throwing objects at her. Mercadel says her neck and back were hurt -- and her finger was so badly damaged ... it might be useless for good.

TMZ broke the story ... sources say Cole went nuclear when she found Mercadel at an early morning party in the penthouse of Cash Money CEO Birdman -- Cole's rumored bf at the time. For her part, Mercadel says in the docs she was just "visiting a friend." Sources at the party tell us there was no banging going down.