Keep in mind that easiest is not necessarily the least expensive option. That said, realistically your two options are to fly or take the train. Either way, you will have to go through Munich. The train from Paris to Munich is around 5+ hours, and you will have to transfer at least once if not twice, and then there will be another hour or so train ride to Regensburg. So the total amount of time you'll spend is up to 8 or more hours (and that doesn't count time waiting to board).
Or you can fly. There are non-stops offered by Air France that take about an hour and a half and cost a few hundred dollars. Keep in mind that you will have to be at the airport two hours ahead of time, but it's still a lot less time than taking the train.
I like taking the train, but if it were me, I would fly, especially since you have to be there early on December 11. Flying gives you more options and definitely speeds things up for you.