To be honest, Porto's recent victory over Arsenal was largely due to luck; Arsenal's performance flagged as the game was about to end, Otherwise I doubt they could have scored in that match, as Arsenal played with great intensity in both the first and second halves. Everyone had a hard time seeing a goal because they both thought the game was really challenging.
However, given how well Porto has played Arsenal at home, I don't think they can duplicate that performance in the second leg. That being said, even though Arsenal is formidable at home, Porto will undoubtedly find the match extremely challenging.
This big competition is indeed very challenging for anyone who plays in the Champions League. There is not a single club that underestimates its opponents in any round in the Champions League because sometimes clubs that are not favorites are able to provide surprises, we also don't think that Porto can beat Arsenal because as far as I know Premier League clubs are very strong, therefore Arsenal will avenge their defeat in the second leg because Arsenal performance at home is no longer in doubt in winning the match.