Domestic Violence
When facing charges for inflicting Injury on a (former) spouse, cohabitant, or dating partner, the defense that generally is most successful is self-defense. The DA has to prove, beyond a reasonable doubt, that you did not act in self-defense.
Domestic violence cases, in most cases, occur at the home, behind closed doors. Often, the DA lacks any other witness to the alleged violence other than the complaining witness (or alleged victim). The DA will thus look to any other circumstantial evidence to indicate who was at fault, logically. For example, prosecutors will examine the injuries of each party, the statements of each party (did you admit anything?), whether someone called 911 and sounded distressed (that call would likely be played in court), and, most significantly, the criminal history of each party. If you have prior arrests of convictions for domestic violence, most likely that would come out in evidence, which often significantly impacts how a jury leans in a "he-said-she-said" case.
Other Defenses?
You could prevail on a more technical defenses. There are a few. For example, was the alleged victim a former lover? The DA has to prove the alleged victim is, in fact, someone you, at the very least, lived with or had sex with at some point or another. If you denied that was ever the case, that could be a defense in itself. But, note, you could still be on the hook for assault or battery; just not the domestic violence variety.
Another example of a technical defense is whether a "traumatic condition" occurred. In other words, the DA has to prove there was, in fact, an injury caused by the direct application of physical force. However, this is not a high hurdle to jump over. Any minor injury, including a small bruise, meets the DA's burden. But, they still have to prove you caused the specific injury by a direct application of force. If there is any doubt as to how a certain injury occurred, whether it was preexisting or caused by something else, then the DA hasn't proved the case.