GBH (without intent) can be tried in either the Magistrates’ Court or the Crown Court. If tried in the Magistrates’ Court a case such as this is likely to last at least a day, possibly longer. In the Crown Court it would probably stretch to 2 or 3 days. You have provided 2 or 3 paragraphs of very brief details, really only an outline of the allegation. Nobody on here could forecast the likely verdict given by a bench of Magistrates or a jury with so few details. As Eddie has said, each witnesses’ evidence will be meticulously examined by each side and presented to the court. The outline you have provided is simply insufficient for an informed answer to be given.
As Eddie has said, in view of the alleged victim's lengthy record for violent offences the defence may apply to introduce what is known as “Bad Character” evidence. This is to put to the court details of his offences to enable the court to draw conclusions as to his nature. This may then be taken into account when considering the credibility of his evidence. The judge or Magistrates have to agree to this evidence being introduced before it can be put before the court.