With Fire and Sword

Oh man, this is a pretty ambitious piece by you. Kudos to that!

Now, time for critiques.
Overall, I like where you going with this. I could see that it takes a lot of time to finish this. The idea behind the composition and values are nice, and it is easy to understand what is going on in this piece. I also dig the details on the woman and the monster. The armor in particular is nicely designed. There's also an easy to understand story-telling as a piece, and you have worked it nicely.

Concept and theme wise, it's a bit of a miss for me personally. The creature looks like it belongs in a sci-fi, the woman looks like she belongs on a fantasy/ medieval setting, and the background suits the post-apocalyptic thing more. But this is pretty subjective. The mixed concept and theme can be successful, but you need to either really sell the 'different worlds clashing together' idea, or try to think about the cohesiveness of the piece as a whole. For example, if it's more of a sci-fi setting, maybe you should imply that by mixing her armor with a bit of sci-fi elements, or make her carry a small, sci-fi-ish gun with her. This would imply that maybe the sword is more ceremonial in nature, and due to circumstances, she needs to use it. I could see that it is implied slightly with the grenade and the bag on her back, but you should make it more apparent in my opinion. The ruins in the background could also use a touch of sci-fi to make it more cohesive since it feels a bit 'mid-age middle eastern' kind of feel. Maybe put in wrecked futuristic public transportations or flying cars, for example. But this is subjective, and I only give you some ideas to build on. Use themes that you feel is the best, and build on that.

Regarding the visual aspect, it also needs more cohesiveness as a whole. The women can be seen drawn using lines, while the creature is more painterly. Pick a style that you can stick with the whole piece and go with that. I actually quite like the work you have done on her costume, and it is pretty detailed. It's very nicely done. But, her sword comes off as a bit cartoonish. One tip is to avoid making props too big or it will appear cartoonish. It doesn't mean that you should make it too small however, but try to go with real life proportions. This is also because it is ingrained into our minds that katanas are slim and light swords so when it comes off that big, people will notice. This, or push it so it becomes really huge and the audience will notice that it is intentional on your part. Think of JRPGs and animes. There's also the issue with the sword looking a bit wonky, but I believe that it can be fixed easily. I feel a StarCraft and Starship Trooper kind of vibe with the creature design, and I think it works. But I can't comment further since I'm not that good myself at designing creatures, lol.

For the finish, the values work out nicely, I think. In the context of the piece overall, try to avoid blacks like on the women's armor. It throws off the value relationship of the whole piece since the dark/black values are too harsh on her. But the form and volume reads okay. One thing that you need to put some focus into is the color cohesiveness of the whole piece. Try to limit your color scheme used since it comes off a bit random here. Try to do some color studies in thumbnail format before proceeding. Look at photos, other paintings, your surroundings, or even stuff like Kuler if you must, and try to choose the best color scheme that helps you sell the idea and intention of your piece clearer. You have Gurney's book, right? If so, the part about color mapping falls into this. The piece comes off a bit random, especially with the purple creature, red woman and green rimlight going on. On paper, it might work since it is complementary, but that's what initial color studies are for! You should also design your colors to add the mood of the whole piece and to highlight the important subjects in a piece.

Okay, I guess that's enough for now. So, what is the main thing we learn here?

'Cohesiveness'

When there's a strong cohesiveness in a piece, the concept and the visual aspect, will be a lot more stronger, more impact, and a lot more memorable. There's something about cohesiveness to humans that makes it feel good. It is more organized, more easily remembered, readily understood, and speaks to us the loudest. There's more to talk about like your rendering ability, but I am confident that you will pick it up quickly as time goes on, but for now, strive for cohesiveness. Tips for producing a cohesive piece that I can give is this. Try to write down your ideas in your sketchbook and read it aloud. It doesn't have to be too detailed of a description, but should focus on a selected theme. Then, do quick thumbnails to create composition, values, and colors. You learn this quickly if you do complete pieces in traditional as there is a very small room for mistake. Do this if you must. And feel free to sacrifice on minute details of certain things for the sake of the cohesiveness and theme as a whole.

Anyway, I hope to see more progress from you, and this looks promising. Take my advice with a grain of salt since 'there are no rules, just tools' (there's a Craig Mullins' sketch that intentionally break the usual composition rule that successfully emphasize the chaotic nature of war, which highlights how true this mantra is). Study more, and actually do a lot more experimenting. Make mistakes, as you learn most from those. Good luck!