1) Use Female Ninja. She doesn't do extra damage against females.

2) Use Barrier Mystery Ninja. Removing her barrier not only removes a significant damage buff from all water/fire ninja for the first three turns, it buffs your own.

3) Use Ninja that that are immune to status. They can't be poisoned, ignited, acupunctured, or immoblized (be careful with your mysteries though, they will be blocked if you get acupunctured/immboilized if you try to use them and fail).