One thing with premier League fans is that they underrate other leagues too much. I remember when Manchester United was about to play Villarreal in the Europa final some years back, all United fans and former players said it was a walk over. Maybe if they had taken time and watched other leagues they would have known better. Porto are not pushovers. They won't go to the Emirates to make things "easy" for Arsenal.
I know Arsenal are favourites but saying they'll win easily might not be true.
I don't know how easily with Premier League fans underrate with other competition after seeing the first leg match result Arsenal defeated by FC Porto and the poor statistic without any one shots or target made by Arsenal. Have difference level and atmosphere for every teams when participating in Champion League and most of team want to reach higher level round to earn much commission. I think Premier League competition in this season have been over after seeing Manchester United and Newcastle United failed in group stage round and left Manchester City are qualifying to the next round.
Porto have capital winning one goal on the first leg, away match to the Emirates Stadium won't get loss and try to defend their winning one goal for qualifying to quarter final as the only one of Portugal League participants.