Weiss: You MAY be overusing ellipses. Granted, some of it came from Sans' speech, so those I can overlook, but I AM seeing a lot of them and it's a bit distracting from the story. Seeing some minor spelling and capitalization errors here and there. Nothing major, but these are things that could be avoided via a Wordcheck. In a few places, I felt like you could have worded things differently to improve the mental image you were hoping to project, but once again it was minor.
All that said, you had excellent imagery in your writing and your motives came through clearly. You clearly paid attention to your opponent's stats in comparison to your own. You toughed out his speedy, but weak, attacks and had trouble hitting him. A particular strength you showed was in your internal turmoil.
Sans: I'm not seeing a lot of wordage variety in this post. Not a bad thing, but thinking you might look to including some synonyms in your writing. If you used "girl" in one sentence, perhaps "opponent" would have been better in the second. Like Weiss, I'm catching some mispellings here and there, but nothing too major.
As with Weiss, you had excellent imagery and motivation. I felt like it came through a bit more clearly in your writing than in hers, particularly in describing the arena you were in.
Both: Neither of you went over your word-count, so yay!
Ok, so this is how I grade.
Enjoyability: (1-10) Did I like what I was reading, or did I end up skimming? How much did you engage me? Did you have a lot of mispellings or grammar errors which made it hard to read?
Penalties

0-Whatever) Did you go over wordcount or miss a deadline? Pretty much the only things I look for in regards to this category. -1 point for every infraction.
SP: Every SP you spend during the fight gives you 1 point in this category.
So, here are my grades.
Weiss
Enjoyability: 8
Penalties: 0
SP: 2
Total: 10
Sans
Enjoyability: 8
Penalties: 0
SP: 1 (Sorry, you used the first SP in the Intro Post. I only include SP you spend during the Combat Posts, even if the effects of the spent SP go into the Combat itself.)
Total: 9
Weiss is the victor, in my opinion. That said, it was extremely close and if not for the SP category, it'd probably have been a draw.
I dunno if Staff give rewards for these OOC fights, but if they do, I'd recommend granting 200 OM to Weiss and 100 to Sans, with Weiss getting the extra as a "victor's" prize.