Arsenal still got a chance to play and get pass this stage of the champions league and also at the same time they are at the verge of loosing it to the Porto too because if they allow their self not to in the second leg, they may stand the risk of not been able to continue with the champions league again this season and it will be a good achievement for Porto because I'm sure they wouldn't believe they would make it to this point.
There are many possibilities and scenarios that could happen in the second leg, you can see in the picture below, Arsenal is the favorite with only @1,24 odds. But we know that Porto have an advantage on the aggregate side which makes them more confident in playing defensively.
Well, with all of these and as I said before, I wouldn't bet on this match because the risk is too high.
If Arsenal plays better than the EPL in the second leg, then the match in the second leg can still be won. They just have to be clinical, not make mistakes and not play EPL-style football in the Champions League. I still bet that Arsenal will win in the second leg, even though Porto are very good at defending. This was proven in the first leg. Arsenal didn't even get a single shot on target. However, because the second leg will be played in the Emirates, of course with the support of their own supporters, Arsenal will be able to dismantle Porto's strong defense.