I am an active, fast-paced reader, capable of focusing on both the smaller details and also the metacognitive aspects of your writing. I assure you, should I take your request, you will not be disappointed. I am less the grammar police and more the ...associate professor when it comes to betaing.
If fanfiction is serious business for you, in that you're interested in telling a story. I am your man.
Credentials include graduate-school level reading and writing - written too many papers and whatnots to count. Writes frequently for various journals at school, and I also edit professional papers for my school's academic publications. I've also studied the structure of fantasy and have some knowledge of general construction based on scholarly essays.
Of course, I still keep an eye on my fandom, and is currently running a blog/site on Fire Emblem analysis and characterization.
I expect to see something that's your best effort. My critique is harsh, and unrelenting. If it's badly done, I will tell you without honeyed words or "well, it's not that bad..." I do this not to injure your ego or to injure your pride, but, rather, consider me a surgeon. I would rather operate on a salvageable fic than to give you the post mortem - after the fic has been published.
That being said, I'd like to point out that I'm one of the most responsible editors out here. Give me a working timeframe and a deadline, and if I take it, I'll get it done. If I see something that I disagree with, I will give you a full rundown, addressing the issue from multiple ways.