Sorry for the late reply.
Great work! I think you have the lighting side pretty much wrapped up, I will only say that you should watch your hot spots on the horses, they are clipping a bit too much. The white horse is reflecting light 'like metal' in places, you loose the detail when it is burning out plus it is a bit distracting from the overall model. I would consider wrapping the key around a tiny bit more to accentuate the lighting on the model.
I really like the idea of using atmospheric lighting for this kind of project. The way you have it set up at the moment looks more like a lens flare than god rays, I actually think that in this instance, a lens flare would be a perfectly good way to achieve the same effect, so maybe do some research into this. The position of the bright hole in the clouds means that the god rays have nothing to contrast against, before it meets the character, see the reference for examples. Also it's not really working because of that particular angle of the light, and the fact that it is probably a bit too subtle. If the background and framing were different, perhaps for different shots, I am sure you could get it to work, to achieve the mood you are after.
When I look at your image, the overall impression is great, its atmospheric, it has drama, but it is being undermined by the strong depth blur on the distant trees. It gives the illusion that the model is small, rather than the true scale of men on horseback, and I think that it is compromising the epic nature of your pose and lighting. That amount of blur would work on a close up, but not a wider shot. If you are going to use depth blur on your images it is best that you have a basic understanding of cameras. I am no expert but I have put together an example, to show you what I mean.
I used a Canon 350D, focal length of 23-mm ( I am using a wide angle lens on purpose) and f-stop 4.5 to take the pictures below . The only variable is the distance of the subject, my WALLE toy, to the camera. The nearer the focusing object is to camera, the shallower the depth of field (DOF is the area in focus). Notice how close the object is, to get the background to blur extensively [that is my arm and I am looking through the viewfinder at the same time]. Try using this calculator to see it in numbers ...
With these settings, the subject only has to be 6.5 meters away for the depth of field to be infinite. Therefore the blur would be over a very big distance, and would be minimal. Try imagining a couple of horses there, how close would I have to get to match your depth blur. You would need a very wide angle lens to fit the horses in the shot and be close enough to reduce the DOF to that extent. With a narrower field of view you would need be substantially further away from the subject and have a super fast expensive lens (the f-stop would be smaller) to match your blur.
Of course depth blur can be used to give an epic feel, but only when the character is supposed to be tiny compared with the background.
I have a pretty good idea why you want to blur the background. To get the painterly feel you are after, to bring the focus onto the model (your lighting and central composition do that already) , to hide things in the background that you don't want people to see. I realise that you have your reasons, but I want to show you how it is going to be seen by professionals and audiences alike. They might think that it is a lack of your understanding guiding you rather than artistic reasons.
My final comment is on the textures on the ground. The grass seems out of proportion with the horses, a little small I think. It's best if the textures match as much as possible to really convince the audience of the scale of your model.
I hope you feel that I have helped you through this mentoring process. You have shown that you respond well to feedback and that you have an artistic flare, very hirable qualities.
Good Luck with the final hand in!