Even though Arsenal applies the same energy they are using to play in the UEFA Champions League to their remaining matches in the Premier League, they still won't be able to close in with Liverpool in the Premier League table because Liverpool can't drop the points that will allow Arsenal to overtake them in the Premier League table and win the Premier League title. Arsenal should leave chasing Liverpool for the Premier League title because they have already lost the battle; they should focus on the UEFA Champions League so that they can knock out Real Madrid and be close to winning the UEFA Champions League trophy.
I agree, putting all their energy into UCL is far more important, considering they are up by three goals Arsenal should be able to get a win here, there isn't really any big deals out of this and could be very important to fix this as well.
If they could get to next stage, that is semi finals, which means there will be only 3 games left for them to win the title, and they could even lose one in theory. How? Well, after they reach semi finals, if they win the first game 2-0 but lose the other 1-0 that means they still go to final game, and then winning that also means UCL champions. So Arsenal needs to put all the efforts they have into next game, to make sure they can go to semi finals, it is going to be crucial for them.