Some nice sounds.
To be honest, I really dislike Slayer. It just usually doesn't sound very good; it's too raw, especially without any effects.
If you're going to use Slayer, try to alter it as much as you can. If you really work at it, you can get some unique sounds with it (though you had some nice stuff going on with the alternating instruments and delay at points).
The same goes for your other instruments. Try to add effects to them to make them more distinct and different. You'll see that a world of possibility opens up with effects :)
Try to stay away from using the FL default drums, too. Especially if you're not going to alter them. There are some better FL drum samples, but you should really try to make some samples for yourself or find free ones on the net.
My favorite sound in this piece was the stringy pad that came in at the beginning. It has a very beautiful, soft feel to it.
Overall, not a bad piece at all, but with some touching up on effects, it could be awesome. I still think it kind of worked though, since it's a video game track.
You get a five for support :) Keep working at it. You've got good potential.