Now there is no other choice, Arsenal must focus on the Premier League and Europa League. Those are two competitions they are still likely to win this season, but I don't expect Arsenal's march to the title to be easy.
I don't think Arteta would have any focus set on the Europa League, if they prolly were in a bad position on the league table with a possibility of not making it into the top four, then they would have concentrated on the Europa League and tried to win it so as to get an automatic ticket into the CL next season.
Having said that, Arsenal are in a great position in the EPL, they currently sit at the summit, thus they would definitely have their focus solely on the EPL, this is a title they haven't won in over 19 years, i think Arsenal's goal is primarily the EPL, the Europa League comes after it.