In the champions league; there are no favorites in the group stage; anyone can beat anyone, anyone can defeat anyone. Also in the champions league; there is nothing like league form, you can be on a 20 game winning streak in your league; and get defeated in the champions league. Arsenal needs a top performance to compete in the champions league.
That's it; any club can lose in the Champions League. While PSG's league play last season was truly outstanding, when it comes Champions League their stand was not nearly as strong even with Messi and Neymar remaining on the team. With people still underrating Manchester United based on their early-season performance, that they will be defeated by Bayern Munich, I believe that even the clubs that are not that good in their league but qualified for the Champions League will be very serious with any team they faced. Munich is a very excellent club, and when it comes to the Champions League, they usually take their games seriously. We shouldn't consider that this season would be any different because, in my opinion, they don't have a formal coach.
Since Arsenal has been performing well this early of season, I believe that they will advance to the next round. I don't think they can lose to any team during the group stage.