The longer you have the child in your custody, the worse her chances are. Because delay works in your favor, your attorney may want move as slowly as possible so the child will be with you in an 'established custodial environment' for as long as possible before the judge has to make any decision. After a certain amount of time (6 months to a year) it almost won't matter, the judge will be reluctant to change the status quo without a very good reason.
You will want to be able to show that the child is doing well in your care, by documenting the child's progress, grades, health, etc.
You may also want to verify that there have been no further incidents of domestic violence in her home, which would certainly count against her re-obtaining custody.