Is there a single word to describe "embrace challenges"? The full context would be,
A man who loves to [embrace challenges].
This isn't quite "one word", but a "go-getter" comes to mind: http://www.thefreedictionary.com/go-getter
I also tend to think that "ambitious" people do not shy away from challenges. Opportunistic also comes to mind.
I know you asked for one word, but the person you are describing, I believe, is simultaneously an ambitious and opportunistic individual.