Techniques for Squarely Cutting the End of a PVC Pipe
There is a tool that is made for cutting out the PVC pipe in a joint so that it may be used again. That tool may be useful to square up the end of the pipe in this case if you were able to hold the tool inline with the pipe while it shaves off the high part of the pipe. This is what one example of the tool looks like.

  

These are used to chucking up the tool into a good sized electric drill (AC powered is recommended). The round slug on the end of the tool keeps the cutter centered in the pipe so that the cutters will shave off the PVC pipe in the coupling. In your case this may allow you to square off the pipe in your drain if you can hold the tool steady whilst it shaves off the high part.

Note that not all PVC pipe shavers are suitable to your situation. Some of them do not have the same type of round slug as shown above. Cheaper ones may just have a thin washer style guide to hold the cutter centered in the pipe. Even the above tool type may not work in your instance if the non-square end of your pipe is too angled.

When searching for these the search phrase you want is "PVC fitting saver".