Obviously Arsenal will be strong competitors in the Europa League, besides that there are also other teams such as Freiburg, Lazio, PSV and Manchester United who will give fierce resistance in every match which of course for every team in the Europa League this season. Barcelona have to fight a little harder in the Europa League because I see Barcelona's performance at this time starting to decline a bit, while the teams in the Europa League are still in better shape.
In fact the Europa League this season is more interesting than last season. Barcelona did fail in the Champions League, but don't think they are weak just because they lost to Bayern and Inter. I will look forward to their fight in the Europa League this season, and it would be better if Barcelona met Arsenal sooner. :v
About Arsenal, they are good but I don't think they were very dominant in the game against a strong team. The fact that Laliga teams tend to be strong when they meet Premier League teams, that's history.