I think it was just for dramatic effect! Maybe, if the story was allowed to run, I could have shown my Arbite as being seriously injured as well!
Never got the chance unfortunately.
You might be right about Alpha's. I suppose I could have used another bad guy!!
To be honest you should take everything I say with a pinch of salt because I'm no writer and I have no idea what BL want in these submissions. In retrospect maybe a fighting scene was a bit ambitious? By describing a lot of action happening you could have been forfeiting the ability to really set a scene and create a distinctive tone to the piece that would grab the attention of the person reading. Perhaps writing the aftermath of the event or moments before it began could have been more effective- when I read BL stuff the fighting parts aren't what I enjoy reading, it's usually the rest of the book that's memorable.
Again, I'd try to emphasize the huge gulf in abilities between an Astartes and a human. Maybe have the human survive purely because the marine didn't turn his attention to him, or let it be freak luck the guy survived the encounter rather than the marine missing his shots.
I can't stress enough that I have zero expertise in these things and am only throwing out suggestions. I respect that you took a shot at it and I'm sure every unsuccessful attempt at getting something published will improve your chances next time. Apparently that Stephen King book 'On Writing' is great for anybody looking to get into the craft, if you haven't read it you might find it useful next time around. Stick at it!