Manchester City may not be performing well in Europe. However, winning the Europa League title is not easy for Arsenal. Renowned teams like Manchester United, Barcelona and Ajax will perform in the Europa League. And each of these teams has the ability to beat Arsenal. It is difficult to maintain form after a long break. If the bonding between Arsenal's players weakens, their performance will suffer. I am looking forward to see how they perform against West Ham in the next match.
Manchester City is playing on Champions League, not Europa League.
I don't expect Manchester United and Ajax will become a winner, Barcelona is more promising since Lewandowski can carry his club. However don't only look on popular clubs only, I think Fenerbache and Feyenoord could be a threat for any clubs since they're have a good strikers. Fenerbache have Irfan Kahveci while Feyenoord have Danilo and Gimenez.