I am not at all worried about Barcelona's form in the Champions League. Except for the first match, they have won every match in the Champions League exclusively. On the other hand, it would be a mistake to think of Dortmund as an easy opponent. Like Barcelona, Dortmund is also in their best form in the Champions League. Both teams have played 5 matches, won 4 and lost 1, and are at 4th and 5th in the points table. However, Dortmund has performed poorly compared to Barcelona in the Champions League. Whereas Barcelona's opponents have played against strong teams like Bayern Munich, Monaco and Brest and have won matches exclusively against Bayern and Brest. On the other hand, Dortmund has won matches against easy opponents. They have not played against any other strong team except Real Madrid. They even lost the match against Real Madrid miserably by 5-2 goals.
Barcelona will definitely be ahead considering past performances. But we are definitely going to see a tough match. 3 points are equally important for both teams to stay at 8th place in the points table.
Seems like you forget Dortmund was up 2:0 in their match IN Madrid. Sure they lost it in the end but they had a chance until the collapse, and it was an away game which is always difficult. Also, Dortmund is a totally different team at home as well. I mean Barca is still the favorite to win this one but the gap is not as wide as you are making it.
Sure Barcelona had 1 more difficult match, which is the one AT HOME against Bayern but the rest is also against soso opponents. Brest is not strong, they are in 11th spot in a league as weak as France. Which good team other than Bayern did Barcelona beat? Nobody!