I'd be surprised if DNA evidence featured in more than about 1 or 2 per cent of assault cases. The vast majority are determined by other evidence. Where it is
entirely one person's word against another's, the prosecution might find it difficult to secure a conviction. However, an ABH charge requires that a certain level of injuries must have been sustained by the victim. See here:
http://www.cps.gov.uk/legal/section5/chapter_c .html#09
So there's usually medical evidence (either in the form of photographs or doctors' statements) to back up the victim's statements. There might also be further evidence, such as CCTV.
The accused person should (irrespective of whether he/she is actually guilty or not) discuss with his/her solicitor whether it's worth seeking a jury trial. If the case does go to the Crown court (either because the defendant has sought a jury trial or because the magistrates believe that their sentencing powers might be insufficient), the defendant should insist upon a case conference, with his/her barrister present, to examine the best course of action. (Far too many defendants never meet their barrister until the day of their court appearance. For the best advice, it's essential to get the defendant, solicitor and barrister together beforehand).
Chris