I don't know if its nature comes into play but it has quite a lot of Defense. So whenever I'm down one Pokemon [Vespiquen] it attacks and attacks with no mercy despite being the second lowest in attacks [Speed being the lowest], it takes all the hits and fires back until the other Pokemon faints. They usually say defense is the best offense, in this case it is because it defends pretty well against attacks.
I think the first one I found was female, I'd totally forgotten at the time you needed a female, haha. ^^; I must have just been lucky first time around, even if I wasn't I'd hunt for hours until I got one.