This UCL season is full of surprises, don't underestimate your opponents, Munich lost against Napoli, Arsenal also lost against Porto. as you said, Arsenal were indeed favorites, even in the first leg they had superior possession of the ball but there was not a single shot on target. So it could be said that Arsenal are still ahead in the second leg, but Porto could provide a surprise. It is very important for Arsenal to never underestimate their opponents if they want to advance to the next round. Moreover, it is predicted that Porto will play defensively and only rely on fast counter attacks. If Arsenal don't learn from the first leg, it is not impossible that the same thing will happen again in the second leg.
I always believe that the UCL league is very different from the domestic league, in the UCL league what is needed is not only strategy but mentality and having the DNA of the UCL league. Currently, the mentality and DNA of the UCL league only belong to Porto, so the second leg for Arsenal will be very tough even though they are playing at home. However, if they make a mistake, Arsenal will be punished by Porto like in the first leg. What is certain is that this match will be intense and exciting to watch.
The knock round of Champion League for this season most surprised after both favorite teams had losses on their first leg match, Bayern Munich unbelievable defeated by Lazio and Arsenal with top consistency performance in domestic league had loss against FC Porto.
Its not close opportunity for both teams qualifying to quarter final if can make come back on second leg match but must win above two goals.
Has advantage with Arsenal and Bayern Munich will be host on second leg match, potential to make come back when playing in front of their fans and have to proof for Bayern Munich as top teams and difficult has eliminate in 16 of round.
Bayern Munich has Champion League mentality winning but I doubt with Arsenal have been absent for several years and get bad capital on the first leg match. Bayern Munich has to play one match in domestic league before facing Lazio in champion league, seems they will make huge rotation against SC Freiburg and most focus to win above two goals against Lazio.