This is cool man. You mentioned it was the most detailed picture you've ever done. I'm not saying that detail is the most important thing, because it isn't, but anytime you push a part of yourself to it's artistic bounds, you find improvement. You asked for a critique, so I'll mention a few things. I think maybe your fireworks are too bright, and take away from the main characters of the image. Next time, maybe you also want to try a different angle on characters, so they are not so lined up in a T formation. A tilted view, maybe from the right side, would be more difficult of course, but it would also make the image much more dynamic. Otherwise, keep pushing yourself like you are and you'll have no choice but to keep getting better Good luck in the judging!
I knew something about both the angle (lack there of) of the character and the fireworks made me heavily speculate what about them both together gave me a bad taste in my mouth concerning this. But you hit the nail right on the head. And agreed on the detail, considering I dislike spending too much time on those things, but it pushed me.
What would be a good suggestion for more angle practice in terms of drawing people/characters?
If you can find a way to go to live figure drawing sessions, it's the best thing you can do. It's worth it to go out of your way to make it happen. Otherwise study the basic structure of a figure (like, knowing the simple shapes) will help you turn and pose the figure in your head. There's such a huge wealth of imagery on the internet too, so just grab figure reference of people in all sorts of poses, and practice drawing them everyday. Another exercise I was taught, was to look at a figure's pose, either from life or a photo, and draw that pose as if you were looking at them from a different angle. That way you still have some kind of reference, but you're using you're pushing your imagination much more.