The first leg has already given a picture of how the second leg will be because we shouldn't expect any comeback from Real Madrid even if they will win Arsenal but the goals won't reach 3 and even if it does, Arsenal will also score one or two goals so the chances of a comeback is not possible for Real Madrid. I think the reason why Arsenal couldn't win their Premier League match today was because they were preparing ahead of the second leg hence they didn't even care about the match against Brentford. Arsenal is a difficult team for Real Madrid and even in their head to head. No matter how strong a team is, there are teams they meet and find it difficult to have an easy win and it appears like Arsenal is one of the teams that Real Madrid finds uneasy.
There is no doubt at this level when you are watching games like these never been easy to have comeback with the difference of 3 goals but when Real Madrid and Champions League added things can take turn. After watching all games now three teams are confirmed for last four Arsenal, Barcelona and PSG while Inter Milan and Bayern Munich still have to give their best because both can go through with this narrow end in first leg game even Inter Milan will be entered with lead and home advantage but still things can take changes.
Now just last rounds of season which are surely important for all teams, and they are managing things for end of their campaign with positive results, but Bayern Munich lost at home against Inter Milan was big setback for them now they have still chance.