It would be an easy 90 minute win for Arsenal. Porto might have a lead over goal but they are no match to what Arsenal is now. The next game is going to be in a much different context as well. Arsenal has placed themselves as the contender for title of the premier league and the match is being hosted in Etihad stadium, the home ground of Arsenal. Arsenal will most probably have a better aggregate than porto during the gameplay and advance to the next round.
Another match on the same day Barcelona vs Napoli would be much competitive one but Barcelona would have a better play.
For Arsenal's win prediction I still agree with what you said because even though they are behind on aggregate now but I am still quite confident that they can win in the second leg later but when talking about an easy win I think it will not be possible because there will be nothing easy in this and they have to try harder if they want to win.
Looking at the first leg alone Arsenal were quite troubled by the monotonous match that took place at the Dragao where they were not even able to get on target once in 90 minutes of the match.
By looking at such conditions even though this match takes place at the Emirates stadium which is sure to be filled with supporters from Arsenal but when a monotonous match like the first leg they play then this still won't be easy.
They will definitely do the same scheme in possession but for the second leg this time Arteta needs to change his attacking scheme because with what was done in the first leg I think it was just a waste of energy without good results.