In addition to defending the allegations in the current lawsuit, if she brought a lawsuit against you based on false allegations and for an improper purpose, you may be able to file a malicious prosecution claim against her if the case against you is dismissed. However, I recommend calling an experienced attorney to fully discuss your situation. You did not provide enough information for your question to be fully answered.
What kind of case is this? You will need to respond yourself of course but also get declarations from any other witnesses who may support your side. But would need to know what kind of case this is to give any more advice.