It is a very tricky game. Indeed, and that's why Arsenal had to give away their Premier League match against Everton for the full preparation of the match against Real Madrid. They know what's really involved in the match, to play against a defending champion. They have to give all their might to win in the first leg because it is as if the Champions League is where their whole hope is, to make a mark in that area since the Premier League is becoming far from their reach to lift it.
Arsenal understands they have not to spare anything in their Arsenal if they must win this game and so they are probably taking it as such considering the team they will be playing against how experienced this team is and how they have set the records in the champions league following they did similar thing that Arsenal did as they didn't really win their last la liga game. Winning this first leg is such an essential for them especially with the fact that they will be playing from home, loosing at the Emirates will feel even worse than if it were to be at Madrid's home, so for Arsenal it's everthing they have gat they must give to win this game if possible.