This piece immediately brought tears to my eyes before I even got a chance to process the entire image. The detail on Dipper alone
The imagery is beautiful-the colors and tone, the shading and lighting, you've worked it all to your advantage. The perspective is a bit skewed, but that may just be the size of The Beast compared to Mabel throwing it off. The Beast is quite a massive being himself.
Speaking of, I greatly enjoy the sharp contrast between The Beast and the rest of the imagery-the black ties him to the edelwood tree that has consumed Dipper. I find it very impressive that you managed to create a dark setting with little use of dark colors. It helped to emphasize the emotion within the piece, especially the expression on Mabel.
Overall, it's a wonderful piece. I'm very excited to go through the rest of your gallery now.