We have seen Munich lost at home several times this season due to their poor form, and that is why Arsenal might be luck to win them at home for this reason. Apart from that, I don't think that it would have been possible for Arsenal to think of winning Bayern Munich in Germany.
Arsenal should see this match as a big challenge for them to use and advance to the semifinals and make record again, because it has been a very long time since Arsenal have qualified to the semifinals. It is a big challenge for them to meet up with maintaining their first position in EPL, and also going to win Bayern in Germany.
In domestic league, Bayern Munich had losses in several matches but its not happening current their performance in Champion League with home match, I think can't comparison with performance of Bayern Munich in domestic league and campion league. Bayern Munich had give up and after defeating by Heidenheim last match Thomas Tuchel have announced give up and said over for winning Bundesliga trophy in this season with many point gap from Bayern Leverkusen.
I can't see easy for Arsenal will win on second leg match, keep focus in premier league next match against Aston Villa to secure the first standings and other way need make rotation to save their players have on the top level performance against Bayern Munich three days later after premier league match.