Basara God, she's so cool. This manga's in my top 5, really. She pretends to be her brother, but that makes her (well, the girl in her) stronger. .
Princess Well, she's not exactly the main heroine, but she has a BIG role in the whole story. Her name's Eshild. She's the toughest girl I've ever seen in a manga (or actually manhwua). It's a serious drama manga, so... Hmm...
Vanilla Ice<--Extremely seconded, this fits your description perfectly.
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It only takes one bullet