What is 女斗士 and when can I use this?
女— female/woman
斗士— fighter
So 女斗士 literally means “female warrior”. You would use this term the same way you use 斗士, since “female” is just an additional detail.
Example:

冠状病毒疫情在全球肆虐，各国护士为了保障人民的安全，都如女斗士般在前线努力奋斗着。
The coronavirus situation is globally rampant; nurses all over the world are fighting in the frontlines, just like female warriors, to ensure the safety of the people.

(I made the assumption that the vast majority of nurses are female)