The second leg of Real Madrid vs Atlético Madrid is definitely difficult to predict as the odds are not very wide. The odds of the two teams are close, but the bookies have tipped Real Madrid to win. Since this match is at Madrid's home ground, they will enjoy the advantage and will be strong in defense and attack. Every match in the Champions League is very difficult, but I think Real Madrid have a lot of experience as they have won the UCL many times. And for Atlético Madrid, Ancelotti will definitely try to adopt a different strategy and build a strong team to enter the quarter-finals.
I still favor Real Madrid to win the match in the second Leg. Although the match will be in Atletico home, Real Madrid seems to have no problem with this. Moreover Real Madrid looks like to have a smaller chance to win La Liga trophy, they will struggle harder in UCL. I'm sure Ancelotti will apply a safer tactic to face Atletico Madrid at their home. Ancelotti and Real Madrid are also have good luck in UCL. So, it is unlikely that Real Madrid to get a lose in the next Leg.
As you simply say, the match is not going to be easy. You have to keep in mind that the match is not being played at Real Madrid's home ground and Real Madrid's performance on the away ground is not guaranteed. However, you can be optimistic for Real Madrid with Atletico Madrid's recent performance. Atletico Madrid is not having a good time at all in the domestic league.
I do not want to increase or decrease the chances of anyone winning in the second leg or knockout match of the Champions League on the 13th. Anything can happen in the match and the result of the match can go in favor of any team. However, I will definitely be optimistic for Real Madrid in the match.