I was planning to take a couple hours of data as the comet PanSTARRS was passing through SH2-27 but clouds limited my data to around 50 minutes. I used 30 mins of usable data here. Not bad for 30 mins of total camera time.

This is how I made an image with fixed stars and comet. I used DeepSkyStacker for that purpose. I tried Astro Pixel Processor for comparison and obtained similar results. This is how I did it with DeepSkyStacker. We want to make two stacks: One for the stars and one for the comet and then blend them together later. Lets call these two stacks the  star-stack image and the comet-stack image. In both cases, we want to make sure to use the "same" reference image for registration and stacking. In DSS, you can right click on a light frame and use it as a reference frame. This reference frame could be the first, the last, the midpoint or any other frame but you should use this reference light frame in generating both the star-stack and the comet-stack images.

For the star-stack image, treat it as a normal stack. Go ahead and register and stack the light frames with the chosen reference frame. Use Kappa-Sigma clipping with the default values as the stacking mode for light frames. You will end up with an image with fixed stars and likely a faint trace of the tail of the comet. That is OK. 

For the comet-stack image, use the same reference frame again. Go ahead and only "register" the frames first. Do not stack the frames in this step. When the registration is done, you should mark the position of the comet on the first, the last, and the reference light frames. DSS will figure out how fast the comet moves in other frames using the time information in the images. To mark the position of the comet, double-click on the first light frame. On the middle right side of the DSS window, you see four icons. The third from the top (the green one) is the "Edit Comet Mode" icon. Click on it. Then zoom on the image and click on the position of the comet. Then save it. The save icon is the forth icon from the top on the middle right side of the window. Do the same thing to the last light frame and the reference light frame, that is, mark the position of the comet in them and save it.  In all case, if the image is too faint and you can not see the comet in the image, stretch it first. Use the bar on the top-right corner of the DSS window and push the middle bar to the left to stretch the image. Now, you should stack the frames. For stacking mode of the light frames, use Kappa-Sigma clipping with the default values. You can use a lower value for the Kappa but do not overdo it as it may kill the comet and its tail as well in the final stack. For the comet mode, under stacking parameters and comet tab, choose "Comet Stacking". Now, go ahead and stack them. The final image should include the stacked comet frames with a fixed comet and likely some faint leftover traces of the stars, particularly around the brighter ones. 

I blended the star-stack and comet-stack images with GIMP. You can similarly use Photoshop. I want only the comet in the comet-stack image to be included in the final blended image. For that purpose, I made a mask. The role of this mask is to make sure that only the comet in the comet-stack image is included in the final blended image. I used the green channel of the comet-stack image as our mask. The reason is because there is better signal in the green channel and more importantly, the comet and its tail are very bright in the green channel compared to the other channels, whereas the red nebulosity for example is almost absent in the green channel. If there are any artifacts in this mask, you can use "clone" tool in GIMP and clean those areas. Now open both the star-stack image and the comet-stack image as layers in GIMP.  Use the star-stack image as the bottom later. Add the mask you made to the comet-stack image (top layer). Now play around with the level and curve adjustments to determine the level of blending you want from the comet in the final blended image. With this mask, you can boost saturation in for example the blue and green channels as well since the coma of the comet has a nice greenish, bluish color. You can do all kind of stuff to the comet with the mask on as well. There are other ways of blending images using masks in GIMP, for example using the paint brush tool but I have better control over the final product with this way of masking I just explained.