First, I think you would probably benefit from tracing your sketches or cleaning them up, as your lineart tends to be rather messy and it detracts (I think) from the overall piece. One thing you might consider doing is work on a canvas 2-4 times as big as the size you plan for the final piece to be and then resize it at the end. It helps minimize any flaws that might be more noticeable otherwise. You also might try a lineless painting, that's always super fun.
Another thing I notice is that your figures tend to be floating in space, which is fine when they are supposed to be (like with Volcorona), but it is a bit off-putting when looking at, say, the Heliolisk one. I'm guessing that you probably draw the object the piece first and then add the background? It might be helpful to work on the piece with the background in mind the entire time, even if you don't work on the background immediately. Doing so will also probably help you with perspective, which you seem to struggle a bit with. For example, in the Flygon piece, the cactus, tumbleweed, and surrounding hills appear to be extremely close to Flygon. Studying linear perspective might possibly be helpful for that? Okay, moving on. You seem to have a pretty good understanding of colour and form. I might suggest, though, posing your figures in more dynamic positions, which might make your images a bit more interesting to look at.
Okay I also have some corel painter specific advice:
Avoid the F-X brushes (e.g., fairy dust and distorto). At least in my experience, these brushes almost always make an image look worse. They make really off-putting patterns that typically never fit thematically with most artistic styles. This also applies to the particle brushes. In fact, it's often best to stick to one or two brushes for most images (that aren't too complicated). I like brushes from the oils, chalk, and pencil categories, as well as custom brushes. I recently received some advice to avoid even these textured brushes and learn to paint with simple round brushes, which I'm currently trying. So that my be a good thing to experiment with.