Although not perfect, really good.
I'm not going to lie, the animation wasn't perfect but it is much better than anything else I usually see here on newgrounds. Also the plot and the story is weak, but Halo doesn't really need a plot, it is basically shoot some aliens, blow shit up and save the day and you capture that very well in this flash. So overall I think it was great, and I'm glad tha t people like you are still submitting things like this which you've actually been working on, and not just thrown together.
But one thing that would have made it a little better than it already is, is that you could have put in some sound effects. The music is really good for building up a good atmosphere, but if add gun shot noise when the main hero is firing his weapon, well that would have made this truly awesome.