What is a word that means "a person or thing which is small yet capable of defeating big giants"?

What can I call a person or a thing that is very small yet capable of defeating big giants?

I need different words that can be connected closely to this meaning.

It can be related to success,war, or anything else.

David is often used in reference to David and Goliath.

Story here.

Edit based on comments: They are right. It is all about context. In many contexts you would have to explain that the person is a David trying to bring down a Goliath. I have though seen the term David used alone when there is a build-up. If you were talking about someone trying to take out giants and said, "He will act like David with his slingshot..." There are tons of ways to do this. Was not implying that just simply mentioning the name David is proper, although some people would still get the inference.

"Underdog"

Typically means "a competitor thought to have little chance of winning a fight or contest", but modern society almost always attaches the underlying concept that the underdog is capable of beating the giants, and is in fact cheered on to do so.

Giant-killer is also used. It seems a bit on the nose, but it is used in that sense.

cf. Jack the Giant Killer.

Small but mighty

Small but mighty
Small but mighty
When you're powerful and wise,
You can rise above any size

_{~ Source: http://disney.wikia.com/wiki/Small_But_Mighty}

There is a rhyming version also: "tiny but mighty"