A N00b Pwner is a person who is good at pwn'ing n00bs (unskilled person in video games) and other players.
He/She is generally skilled in games, so they can pwn other people. (esp. in Online Games)
"Did you see Jake's Guild in the GvG and how they pwn'd those n00bs?"
"They're called N00b Pwners, so go figure."