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My first thought is that you should get up in front of a mirror and try out that pose for yourself while pretending that you're thrusting a very heavy sword. The lifted elbow and flailing back arm don't seem to fit with this situation; he looks more like he's falling out of a window. I'm also not really sure what the main light source is. I see a moon or something behind the creature, but that couldn't possibly be illuminating the creature's torso and the boy's face.
This comment is more about future projects, since you're already pretty involved in this piece, but you should really try out different viewpoints and compositions to find a more interesting composition. This piece feels like you had an idea for a battle and just starting sketching it out from the most obvious point of view without really thinking about how to play up the drama or make your hero's plight clear.
Thank you for the wonderful crit @Dierat! ^_^