Dortmund was able to reach the finals last season, but got defeated by Madrid. Dortmund is very experienced in UCL and I believe that they will go far this season. Barcelona is in their best form and one cannot be underestimated, because they have scored a lot of goals this season. This shows that their attack is very active. However, the match is gonna be one of the best in the round of 16.
In the round of 16 of the UCL this season Dortmund will face Lille which is a team that is not so strong in their domestic league so it could be a good opportunity for Dortmund to advance to the last eight of the UCL this season. However, I don't understand why Dortmund are not so good in the Bundesliga this season when Dortmund should be able to put themselves in a better zone than now so they can return to the UCL competition next season.
Dortmund started this very badly because their coach did not have the quality and experience but after the improvements that have occurred now Dortmund's performance has at least improved and we can see that in Dortmund's last two matches especially their front line which is productive again.
For the competition in the Champion League competition is indeed difficult but Dortmund must still be taken into account because they have a good mentality in the UCL even though they could not win the title but were able to reach the final last season as a reference for this season.