If Dortmund's luck is with them, they can beat Real Madrid and qualify for the title, but performance-wise, they can never beat Real Madrid.
Dortmund have to prepare well in tactics with multiple tactics for possible changes in the final match, then when they go to play the final, they have to fight seriously for chance to win the match. Luck is important but it won't come if Dortmund won't seriously play to defend their goalkeeper and to find winning goal.
Because of Real Madrid's performance from the start and their strong star-studded squad, they are not easy to beat when they enter the final. If Dortmund play according to their current performance, they will never be able to match Real Madrid.
Real Madrid are stronger and more experienced than Dortmund but with some shocks created by Dortmund in this season, they are actually coming nearly to a happy ending for an underdog. Real Madrid must analyze the adventure of Dortmund in Champions League so far and figure out what are weaknesses of opponent to exploit it in the final match.
Dortmund don't have to fear anything about Real Madrid, at the final match, they must be confident in themselves and take advantage of any chance to win it.