How do you beat the case? First, hire a decent attorney.
That attorney should work up the case to the best of his/her ability. Talk to the witnesses, review all of the evidence, come up with the best arguments possible. There is no guarantee to beating a case. Your best bet is simply to have an attorney willing to do the work and give your loved one the best fighting chance possible.
As far as one witness being enough, if that witness provides enough info in a credible fashion for a jury to find that the defendant is guilty beyond a reasonable doubt, then yes, it is enough.
I agree with Mr. Tuthill... a lawyer is unable to tell you how to beat an aggravated assault case via this forum. A lawyer has a lot work to do before they can properly defend someone on a second degree felony that the range of punishment is from 2-20 years in prison.