Don't forget that there is no difference between the performance of Lille and that of Dortmund this season in their domestic league as a matter of fact both teams have achieved better results in the champions League than their league so I don't think Dortmund will have an easy match against Lille if they try to underestimate them they may end up losing the game and Lille advancing to the round of 8. Dortmund is doing well in the champions League which is the only thing I can say and winning the champions league this season should be their target since they couldn't do it when they met Real Madrid in the finals, if they are able to show good performance I think they will have the advantage of defeating Lille.
You said meet Real Madrid in the finals! Dortmund can't meet Real Madrid in the finals because non of the two teams is even guaranteed a place in the Quarter finals yet until they play out their two games first. Real Madrid is facing Atletico Madrid, a team they have been struggling to win for some time now. if the get past ATM, they would still face Arsenal or PSV which might be easy for them though but what about PSG or Liverpool? Dortmund on the other hand will have to face Bayern Munich or Bayern Leverkusen and then Barcelona. Can they survive it?