Arsenal needs to come up with a new attacking strategy that they can use to breakthrough into Porto's defense so that it wouldn't be a problem for them to score at home. Arsenal can take advantage of playing at home to win the match if they are not outperformed by Porto's strategy, so that they don't play the game the way Porto wants it. Arsenal also needs luck in that game, because Porto was lucky enough to have a dying minute goal to give them the victory.
However, Arsenal don't go far in the champions league, and I hope that they will put in all their best performance to make sure that they win Porto is Emirate, because they are in a good form, so I don't think that there should be any excuse or reason why they will not be able to win Porto. Maybe we might be underestimating Porto because we just want Arsenal to win, but the facts remains that the club with the best strategy will surely qualify to the next round.
Your quite right about strategy but also players and determination can still do a lot in a match, I don't know arsenals plan concerning the champions league, because I can see them more focused on the premier league than any other league, and I don't think arsenal is well built to survive the competition unless they actually become more determined, no easy win would happen if they don't find ways to create more chances and break ports defense, and I think porto would play a more defensive game in their second leg but we know arsenal knows how to touch the ball in small spaces if you see their last match against newcastle they actually did a lot of 18 yard touches, if Porto plans on playing defense then arsenal might have winning chance, I'm predicting up to 2-3 goals for arsenal.
Now after losing first leg match at Porto things are looking much better, and we have again Arsenal at their best in game against Newcastle United where their attackers done good job and able to score 4 goals now they are going to have few games before second leg match at Emirates Stadium and in these games they can use their new strategy which will help them against Porto, and also they will be able to have better result after this defeat which bring them down, but surely things can take good turn for them.
Now we have to check how things will be work for the Porto because if they will come with their defensive approach surely this could be not ideal after winning first leg game at home they surely also have to be aggressive which will surely help them and also bring Arsenal under pressure which needs to win this game at all cost.