I don't know what your exact preference for music was gonna be, but I wanted to try and contribute something for putting me in the movie.
As far as your story, I didn't know if we would be fighting each other or someone else. But I think the song I'm suggesting is good in any case. The beat is uptempo and fits the mood. If this is indeed a internal Newgrounds war, then the words make sense too.
You've probably heard the song. It is on the Blade 2 soundtrack.
Against I by Mos Def and Massive Attack
Here is some of the chorus and song, so you can actually see if the words fit the story.
"Iya.....I against I.
Flesh of my flesh
and mind of my mind.
Two of a kind, but one won't survive.
My image is reflected in the enemies' eyes
and his image is reflected in mine the same time"
"Right here is where the end gon start at.
Conflict, contact, combat.
Fighters stand where the land is marked at
Settle the dispute about who the livest...
Three word answer: 'Whoever survives this.'
Only one of us can ride forever.
So you and I can't ride together
Can't live or can't die together
All we can do is collide together
So I skillfully apply the pressure
Won't stop until I'm forever. One!"
Well, this is just a suggestion. I'm just happy I'm apart of this. Good luck with this no matter what you choose. I'm supporting it a 110%.