Tough is understatement because both teams have history and they're both keen on facing each other. I think the weaknesses of Bayern Munich is pointed out in the open and Arsenal are formidable this season, Mikel Arteta have watch and observed the strength and weaknesses of the Bavarians while he looks forward to tackle down the Bundesliga giant while Thomas Tuchel already have experience when it comes to English teams, he just known how to deal with them but he should remember that alot have changed, Arsenal is not the same way he feels it is.
Even the mood of the team plays a role, and when the coach doesn't have enough authority, it can complicate things even more. I like Bayern better as a team, but I understand that Arsenal are doing better now, I think the first match can tell us a lot about this confrontation. If Arsenal manage to play in attack as they have done in most of the last successful games, then for Bayern everything can be over very quickly, if Arteta starts doing something like he did with City, then Bayern will have a chance.