C'mon bro, you're not new to football. You can't really believe a sure win is possible in a competition like this. This is the champions league where anything is possible. We've seen giants of the game get knocked out my unexpected teams.
I can partially agree with Arsenal definitely knocking out Porto because Arsenal is in a good form and Porto are really bad currently, but Barcelona and Napoli are on equal form if you look at it more closely. Barcelona have been horrible, probably worst than Napoli who have also been bad. So saying it's a sure win for Barcelona is not true to me. Barcelona has a upper hand, but ist not a sure win.

You must also remember that they matches would be player in February next year, that relatively a long time from now. Teams can loose form or gain from during this period. Players can get injured and coaches can be changed and make the team better within this period.
Barcelona and Arsenal are favorites, but you can't be a hundred percent certain that they'll win.