grammar ninja
An individual who displays sub-par utilization of proper grammar either through oral or written communication; the individual showcases aggressive and offensive mis-use of the English language in a manner that resembles the verbal or written equivalent of a ninja attack: fast, relentless and ultimately damaging.
With her steady flow of "I ain't got no..." phraseology, she was an unstoppable grammar ninja, butchering the king's English during the course of every conversation and ill-worded e-mail message.