Which Ashton episodes are mandatory?

My SO and I are binging through the series and are reaching the end of Charlie's run. I told her that we don't have to watch Ashton's episodes, but she wants to see how the show ends.

I've seen all of Charlie's episodes and most of Ashton's first season. That in mind, I want to make a list of episodes we can watch or skip on our way to the finale. Any suggestions of mandatory or skippable episodes is appreciated.