It’s not permitted in Islam for a woman to fight in a battle field with enemies face to face. And if we have some examples where few women came in the battle field was due to other reason but not to fight with the enemies. Also the first lady martyred in Islam was Lady Somaiyya the Mother of Ammar bin Yasir (r.a) but her martyrdom was not taken place in a battle field. She was killed in Mecca. Of course she was martyred in the way of Islam and Prophet (p.b.u.h) but not directly at defence for the Holy Prophet (p.b.u.h).