There's some interesting things here. You've got several good ideas that just need some tweaking here and there!
I like that you're experimenting with different kinds of framing. Next time try cropping in closer to the subject's face. It's gotten a bit lost among the giant boards, and it's hard to tell where he's looking. You also may want to drop your exposure a little bit.
Putting your subjects on different levels is a great way to add interest to a photo. Nice use of the available space! I'm a bit confused about what's going on, though. Is the boy about to ambush the girl? Try cropping in closer and shooting from a slightly lower angle to increase the feel of a looming threat.
Shooting guns (pun fully intended
) is tricky. Pointing weapons straight at the camera foreshortens them and they can look strange. Try having your subject aim over your shoulder to create a longer line. Also, the expression is odd. I would suggest having him relax his mouth a little bit, into a line. This looks too cute for a gunslinger.
I like this one. The cocked shoulders and straight-on gaze creates quite a bit of attitude! You may want to drop the exposure here too. The whites are blown out. Watch out for little costume malfunctions when shooting - the gun clip got caught behind the jacket.
Since the subject has binoculars, I'd like to be able to follow her line of sight. Unfortunately, too many other lines fight with it. The horizontal bleachers are clashing with the strong angles of the railing, the trees, and her bunny ears. Instead of shooting her here, I would move to a different location or zoom in closer to eliminate the contrasting elements. Also, you could try angling the cosplayer so the binocular lenses are facing more toward the camera.
My least favorite of the lot. It's overexposed, I can't tell what she's aiming at, and the background is chaotic. Shooting cosplayers from behind is very tricky, because you don't have the face to work with. Including a strong element of the costume (like her ears) can help.
Also, there's some stray hairs from the wig all over the back of her jacket.
I think your other shots were much more engaging.
Again, overall I think you've made a good start. Hopefully I've been able to give you some assistance and ideas. Thanks for sharing your photos - I'll be interested to see what else you come up with!