The victim, in his 50s, was slashed across his arm as the gang caused damage to the business on High Street.
Emergency services rushed to the scene at around 9.40pm last night and treated the man at the scene.
He has since been taken to hospital with serious injuries which are not believed to be life-threatening.
One ambulance and two paramedic officers attended, a spokesman for West Midlands Ambulance Service said.
Officers from West Midlands Police are now trawling through CCTV in a bid to establish the circumstances behind the attack.
Anyone with information has been urged to contact the force on 101 or through their website, quoting 20SW/243899J/20.